Base editors with improved precision and specificity

ABSTRACT

Methods and compositions for improving the genome-wide specificities of targeted base editing technologies.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No. 16/615,559, filed Nov. 21, 2019, which is a national stage application of PCT/US2018/034719, filed May 25, 2018, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/511,296, filed on May 25, 2017; Ser. No. 62/541,544, filed on Aug. 4, 2017; and Ser. No. 62/622,676, filed on Jan. 26, 2018. The entire contents of the foregoing are hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant Nos. GM118158 and HG009490 awarded by the National Institutes of Health. The Government has certain rights in the invention.

SEQUENCE LISTING

This application contains a Sequence Listing that has been submitted electronically as an ASCII text file named “Sequence_Listing.txt.” The ASCII text file, created on May 6, 2022, is 95 kilobytes in size. The material in the ASCII text file is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

Described herein are methods and compositions for improving the genome-wide specificities of targeted base editing technologies.

BACKGROUND

Base editing (BE) technologies use an engineered DNA binding domain (such as RNA-guided, catalytically inactive Cas9 (dead Cas9 or dCas9), a nickase version of Cas9 (nCas9), or zinc finger (ZF) arrays) to recruit a cytidine deaminase domain to a specific genomic location to effect site-specific cytosine→thymine transition substitutions^(1,2). BE is a particularly attractive tool for treating genetic diseases that manifest in cellular contexts where making precise mutations by homology directed repair (HDR) would be therapeutically beneficial but are difficult to create with traditional nuclease-based genome editing technology. For example, it is challenging or impossible to achieve HDR outcomes in tissues composed primarily of slowly dividing or post-mitotic cell populations, since HDR pathways are restricted to the G2 and S phases of the cell cycle³. In addition, the efficiency of HDR repair can be substantially degraded before and after edits are created by the competing and more efficient induction of variable-length indel mutations caused by non-homologous end-joining-mediated repair of nuclease-induced breaks. By contrast, BE technology has the potential to allow practitioners to make highly controllable, highly precise mutations without the need for cell-type-variable DNA repair mechanisms.

SUMMARY

CRISPR base editor platforms (BE) possess the unique capability to generate precise, user-defined genome-editing events without the need for a donor DNA molecule. Base Editors (BEs) that include a single strand nicking CRISPR-Cas9 (nCas9) protein fused to a cytidine deaminase domain and uracil glycosylase inhibitor (UGI) (BE3) efficiently induce cytidine-to-thymidine (C-to-T) base transitions in a site-specific manner as determined by the CRISPR guide RNA (gRNA) spacer sequence¹. As with all genome editing reagents, it is critical to first determine and then mitigate BE's capacity for generating off-target mutations before it is used for therapeutics so as to limit its potential for creating deleterious and irreversible genetically-encoded side-effects. Here we propose technological improvements to BE technology that will enable its maturation toward clinical relevance. First, we describe methods for limiting the absolute number of available cytosine substrates available for BE deamination by building BEs that make use of deaminases, either natural or engineered, that can only deaminate cytosines that exist in particular 2- or 3-nucleotide genomic contexts (Table 1).

The proteins can include one or more mutations listed in Table 7, e.g., to increase specificity of deaminase proteins or domains on their own or in any possible combinations. The proteins can include one or more mutations listed in Table 8, e.g., intended to alter the targetable motif sequence, optionally combined with any of the mutations in Table 7, e.g., to create engineered deaminase proteins or domains with altered and increased substrate sequence. Further, the proteins can include one or more mutations listed in Table 9, optionally combined with any of the mutations listed in Table 7 or Table 8 to create engineered deaminase proteins, e.g., with altered specificity for the first or third nucleotide in a trinucleotide motif and with increased specificity for its target motif relative to other possible deamination substrate motifs.

In some embodiments, the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises one or more mutations shown in Table 7, 8 and/or 9. In some embodiments, the mutation is N57A/G/Q/D/E; A71V; I96T; Y130F; or K60A/D/E, or a combination thereof.

In some embodiments, the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises (i) one or more mutations shown in Table 7 and/or 8 and (ii) one or more mutations shown in Table 9.

In some embodiments, the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises hAPOBEC3A with a mutation at one or more of N57 (preferably N57G or N57Q); K60 (preferably K60A or K60D), and/or Y130 (preferably Y130F); or hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H with a mutation corresponding to N57 (e.g., at position 3 as shown in Table 7, e.g., a G at position 3). K60 (e.g., at position 4 as shown in Table 7, e.g., a D at position 4)), or Y130 (e.g., at position 6 as shown in Table 7, e.g., an F at position 6).

In some embodiments, the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises hAPOBEC3A with a mutation at N57 (preferably N57G) and Y130 (preferably Y130F), or hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H with a mutation corresponding to N57 (e.g., at position 3 as shown in Table 7, e.g., a G at position 3) and a mutation corresponding to Y130 (e.g., at position 6 as shown in Table 7, e.g., a F at position 6).

In some embodiments, the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises hAPOBEC3A with a mutation at A71 and/or 196, or hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H with a mutation corresponding to A71 and/or 196 (e.g., as shown in table 10). For example, I96T, A71V, and Y130F each attenuate the editing activity of the deaminase from WT (which is critical because of off-target effects) without restoring much if any sequence preference, making them good candidates for all base editing sites.

In addition, provided herein are methods for treating a subject with beta thalassemia mutation HBB-28 (A>G), comprising delivering a therapeutically effective amount of a fusion protein of any of the preceding claims, wherein the deaminase comprises APO3A comprising a mutation at N57G or N57A or N57Q or K60A or K60D or Y130F, and preferably wherein the fusion protein comprises a ssDNA nicking or catalytically-inactive Cas9.

In some embodiments, the fusion protein is delivered as an RNP, mRNA, or plasmid.

In some embodiments, the methods include delivering the fusion protein ex vivo to a population of cells comprising CD34+ hematopoietic stem and/or progenitor cells collected from the subject under conditions sufficient for deamination of the mutated, and re-infusing the cells back into the subject.

Also provided herein are methods for deaminating a selected cytidine in a nucleic acid, the method comprising contacting the nucleic acid with a fusion protein or base editing system described herein.

Additionally, provided herein are compositions comprising a purified a fusion protein or base editing system as described herein.

Further, provided herein are nucleic acids encoding a fusion protein or base editing system described herein, as well as vectors comprising the nucleic acids, and host cells comprising the nucleic acids, e.g., stem cells, e.g., hematopoietic stem cells.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Methods and materials are described herein for use in the present invention; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, sequences, database entries, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

Other features and advantages of the invention will be apparent from the following detailed description and figures, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1. Diagram of an exemplary typical high efficiency base editing system. A nicking Cas9 (nCas9) bearing a catalytically inactivating mutation at one of its two nuclease domains binds to the target site dictated by the variable spacer sequence of the sgRNA. The formation of a stable R-loop creates a ssDNA editing window on the non-deaminated strand. The Cas9 creates a single strand break in the genomic DNA, prompting the host cell to repair the lesion using the deaminated strand as a template, thus biasing repair towards the cytosine→thymine transition substitution.

FIGS. 2A-2B. FIGS. 2A and 2B show alignments for Recognition Loop 1 (RL1) and Loop 7, respectively (SEQ ID NOs: 1-9). Both regions show low conservation across all proteins and have either been empirically demonstrated to significantly contribute to the sequence specificity of each enzyme or are predicted to do so based on homology. Alignment performed by Blosum62 matrix and visualized in Geneious.

FIGS. 3A-3B. Protein-DNA contacts made between APO3A and ssDNA substrate. RL1 residues are shown in magenta, Loop 7 residues are shown in red, ssDNA substrate is shown in yellow, and the remaining residues of APO3A are shown in gray. FIG. 3A shows the orientations of all residues in RL1 and Loop 7 that we propose to make substitution mutations to in order to alter or enhance the specificity of the deaminase domains described herein (SEQ ID NOs:1-9). FIG. 3B shows the base-specific hydrogen bond, shown as black dashed lines, formed between the thymine at the −1 position (relative to the target C) and D131 (SEQ ID NOs:10-18). PDB code: 5SWW.

FIG. 4. APO3A-nCas9-UGI proteins were overexpressed in U2OS cells with a guide RNA targeting a site within a genomically integrated enhanced green fluorescent protein (EGFP) gene (SEQ ID NO:19). After 72 hours, genomic DNA was extracted from these cells and subjected to Illumina sequencing. The frequency of C>T transitions for each cytosine nucleotide within the spacer was plotted for each of the 15 APO3A variants bearing single residue substitutions.

FIG. 5. APO3A-nCas9-UGI proteins were overexpressed in U2OS cells with a guide RNA targeting a site within a genomically integrated EGFP gene (SEQ ID NO:19). After 72 hours, genomic DNA was extracted from these cells and subjected to Illumina sequencing. The frequency of C>T transitions for each cytosine nucleotide within the spacer was plotted for the APO3A-nCas9-UGI proteins bearing single residue substitutions at N57 or K60. Each of these proteins deaminated the off-target GCT motif within the editing window with decreased efficiency relative to the wild type base editor protein but retained efficient deamination activity at the on-target TCG motif within the editing window.

FIG. 6. The ratio of cytosine-to-thymine editing efficiencies for the on-target TCG and the off-target GCT motifs (both in the editing window) at EGFP site 1 are plotted for each of the APO3A-nCas9-UGI proteins bearing single residue substitutions. The proteins bearing N57 or K60 substitutions have increased specificity ratios relative to the wild-type APO3A base editor protein.

FIG. 7. APO3A-nCas9-UGI proteins were overexpressed in U2OS cells with a guide RNA targeting a site within a genomically integrated EGFP gene (SEQ ID NO:20). After 72 hours, genomic DNA was extracted from these cells and subjected to Illumina sequencing. The frequency of C>T transitions for each cytosine nucleotide within the spacer was plotted for each base editor protein.

FIG. 8. Cumulative cytosine-to-thymine transition frequencies for all cytosines (all of which reside in off-target trinucleotide motifs) within the editing window of the EGFP site 2 target are plotted for each base editor protein. APO3A base editor proteins bearing single residue substitutions at N57 and K60 have significantly decreased ability to deaminase the off-target cytosines present in the editing window of this target site. Notably, APO3AN57A did not show any deaminase activity at this site.

FIG. 9. U2OS cells bearing a stably integrated EGFP gene were transfected with plasmids expressing a guide targeting EGFP (the guide comprises TCAGCTCGATGCGGTTCACCAGGG, SEQ ID NO:22) and the indicated base editor protein. After 72 hours, genomic DNA was extracted from the cells, the target site was amplified by PCR, and the PCR products were subjected to high throughput Illumina sequencing. The frequency of C-to-T transitions at the cognate and non-cognate motifs were plotted for each of the transfection conditions. Mutations made to hAPOBEC3A (APO3A) N57 were very effective in restoring sequence specificity of the deaminase domain in the base editor architecture.

FIG. 10. HEK293T cells were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. APO3A BE3 proteins bearing two substitutions generally lost significant activity at the cognate motifs in both gRNAs tested. However, the APO3A N57Q/Y130F Base Editor 3 (BE3) double mutant retained activity on the cognate motifs of both gRNAs while significantly decreasing the frequency of deamination at the non-cognate motifs.

FIG. 11. HEK293T cells were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. After 72 hours, genomic DNA was extracted, the target site was amplified by PCR, and the PCR products were subjected to high throughput Illumina sequencing. The frequencies of C-to-T transitions for all cytidines within the 20 nucleotide spacer sequence are plotted in heat map format. All three APO3A BE3 proteins were able to strongly bias deamination towards the cognate motif compared to BE3 and the engineered variants YE1, YE2, and YEE BE3 (YE BE3s), which decrease the frequency of such bystander mutations by incorporating point mutations into the rat APOBEC1 (rAPO1) deaminase domain that slow its kinetic rate and limit the length of its editing window compared to BE3⁴⁴. Reference sequences, SEQ ID NOs:23-28.

FIG. 12. HEK293T cells were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. After 72 hours, genomic DNA was extracted, the target site was amplified by PCR, and the PCR products were subjected to high throughput Illumina sequencing. The frequencies of C-to-T transitions for all cytidines within the 20 nucleotide spacer sequence are plotted in heat map format. All three APO3A BE3 proteins were able to strongly bias deamination towards the cognate motif compared to BE3 and the YE BE3 proteins. Reference sequences, SEQ ID NOs:29-34.

FIG. 13. HEK293T cells were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. Samples were processed as outlined before. APO3A BE3 proteins bearing mutations to A71 or 196 in addition to N57G greatly decreased deamination frequencies at non-cognate motifs. The ratios of cognate to non-cognate editing for each site were plotted for all three sites for each of the variant base editor proteins, demonstrating that APO3A N57G I96T BE3 achieved cognate:non-cognate editing ratios of approximately 13 for each of the three sites tested. Reference sequences, SEQ ID NOs:35-37.

FIG. 14. HEK293T cells were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. Samples were processed as outlined before. To obtain the deamination frequencies plotted, all C-to-T transition frequencies for the cytidines falling within the editing window were summed. APO3A BE3 induced very high levels of deamination at 16 of 25 off target sites. BE3 induced deamination at the same off target sites as APO3A BE3, but at lower frequencies. APO3A N57G BE3 induced indels at only 6 of 25 sites, and at much lower frequencies than BE3 at those 6 sites.

FIG. 15. HEK293T cells were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. Samples were processed as outlined before. To obtain the deamination frequencies plotted, all C-to-T transition frequencies for the cytidines falling within the editing window were summed. APO3A BE3 induced very high levels of deamination at 3 of 15 off target sites. BE3 induced deamination at the same off target sites as APO3A BE3, but at lower frequencies. APO3A N57G BE3 did not induce deamination at any of the 15 investigated off target sites.

FIG. 16. Schematic showing the potential allele products from targeting the HBB −28(A>G) disease allele with base editor proteins. Mutation of the −25 bystander cytidine present in the editing window causes beta-thalassemia phenotypes independent of editing at the −28 position. As such, it's critical that the −25 position in not edited by the base editor protein. SEQ ID NOs:38-42 are shown.

FIG. 17. HEK293T cells bearing a chromosomally-integrated, 200 base pair fragment of the HBB −28 (A>G) disease-causing allele were transiently transfected with plasmids expressing the HBB −28-targeting gRNA and the indicated BE protein. BE3 and the YE BE3 proteins deaminated the cognate and non-cognate cytidines are approximately equal frequencies, while the APO3A N57G BE proteins strongly deaminated the −28 cognate motif to correct the disease while avoiding deamination of the second cytidine in the editing window. Reference sequence, SEQ ID NO: 43.

FIG. 18. BE3 and the YE BE3 proteins induce deamination of the disease-causing HBB −28 (A>G) mutation, but this results in a perfectly corrected allele in less than 1% of total alleles sequenced. Conversely, APO3A N57G BE proteins produce perfectly corrected alleles in 15-22% of total alleles sequenced.

FIG. 19. HEK293T cells bearing a chromosomally-integrated fragment of the HBB −28 (A>G) allele were transiently transfected with plasmids expressing the indicated gRNA and base editor protein. Samples were processed as outlined before. To obtain the deamination frequencies plotted, all C-to-T transition frequencies for the cytidines falling within the editing window were summed. BE3 produced deamination at 3 of 8 off target sites investigated, while APO3A N57 BE3 produced deamination at just one of the 8 sites, and at very low frequency. SEQ ID NO:44 is shown.

DETAILED DESCRIPTION

In the most efficient BE configuration described to date, a cytidine deaminase (DA) domain and uracil glycosylase inhibitor (UGI; a small bacteriophage protein that inhibits host cell uracil deglycosylase (UDG), the enzyme responsible for excising uracil from the genome^(1,4)) are both fused to nCas9 (derived from either Streptococcus pyogenes Cas9 (SpCas9) or Staphylococcus aureus Cas9 (SaCas9). The nCas9 forms an R-loop at a target site specified by its single guide RNA (sgRNA) and recognition of an adjacent protospacer adjacent motif (PAM), leaving approximately 4-8 nucleotides of the non-target strand exposed as single stranded DNA (ssDNA) near the PAM-distal end of the R-loop (FIG. 1). This region of the ssDNA is the template that is able to be deaminated by the ssDNA-specific DA domain to produce a guanosine:uracil (G:U) mismatch and defines the editing window. The nCas9 nicks the non-deaminated strand of DNA, biasing conversion of the G:U mismatch to an adenine:thymine (A:T) base pair by directing the cell to repair the nick lesion using the deaminated strand as a template. To date, the DA domains described in these fusion proteins have been rat APOBEC1 (rAPO1), an activation-induced cytidine deaminase (AID) derived from lamprey termed CDA (PmCDA), human AID (hAID), or a hyperactive form of hAID lacking a nuclear export signal^(1-2, 5-7). BE technology was primarily established using the SpCas9 protein for its nCas9 domain (nSpCas9), but although herein we refer to nCas9, in general any Cas9-like nickase could be used based on any ortholog of the Cpf1 protein (including the related Cpf1 enzyme class), unless specifically indicated.

An important consideration for the use of BE in therapeutic settings will be to assess its genome-wide capacity for off-target mutagenesis and to modify the technology to minimize or, ideally, to eliminate the risks of stimulating deleterious off-target mutations. With current generation BE technology, we can predict three potential sources of off-target mutagenesis: (1) unwanted modification of cytosine bases within the on-target site because nCas9-stimulated R-loop formation can expose a total of 8 on-target nucleotides for deamination; (2) off-target R-loop formation (Cas9 has a well-documented ability to bind at off-target sites with varying degrees of homology to its sgRNA⁸⁻⁹) leading to cytosine deamination at these sites; and (3) BE-mediated deamination that might occur at sites without binding to DNA by the Cas9 part of the fusion (e.g., activity mediated from solution or at sites weakly specified only by the deaminase itself). Herein, we described technological improvements to BEs that can be used to reduce or eliminate potential unwanted BE mutagenesis.

Increasing the Specificity of Base Editors by Using Cytidine Deaminase Domains with Higher or Altered Target Site Preferences

Current generation BE technology uses the cytosine deamination activity of rat APOBEC1 (rAPOBEC1) to effect cytosine→thymine transition substitutions in a window of approximately 8 nucleotides of single stranded DNA (ssDNA). The ssDNA target window is formed by the R-loop created by the nCas9 portion of the BE fusion protein and begins approximately 4 nucleotides downstream of the 5′-most nucleotide of the sgRNA complementarity region. Thus, the target C to be deaminated is within the gRNA target complementarity sequence. Within the target sequence, it needs to be located at positions 5, 6, 7, 8, or 9 counting from the 5′.

The rAPOBEC1 domain has little intrinsic substrate sequence specificity on its own and deaminates cytosines in all sequence contexts equally well unless the base immediately 5′ to the target C is a G, in which case deamination is less efficient but still possible. In addition, when rAPOBEC1 is fused to nCas9 it appears to have processivity for multiple cytosines within the editing window¹—a single binding event at the ssDNA target window often results in deamination of more than one cytosine in the window (if two or more cytosines are present).

Multiple deaminations per target window can result in undesired changes at the on-target binding site and at other off-target DNA sequences bound by nCas9 in the genome. To some degree, unwanted deaminations within the on-target editing window can be controlled by changing the length and flexibility of the protein linker separating rAPOBEC1 and nCas9; however, this control cannot be tuned to specific sequence locations in the editing window and still shows detectable deamination outside of the limited editing window¹⁰.

To decrease or completely ablate undesired deamination of non-target cytosines within the on-target site's editing window, deamination of any cytosines at off-target sites, and limit unwanted processive deamination events at the on-target site, we built BEs using engineered deaminase domains with intrinsic specificity for short sequence motifs and/or non-processive deamination activity.

Base Editors

In some embodiments, the base editor is a deaminase that modifies cytosine DNA bases, e.g., a cytidine deaminase from the apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like (APOBEC) family of deaminases, including APOBEC1, APOBEC2, APOBEC3A, APOBEC3B, APOBEC3C, APOBEC3D/E, APOBEC3F, APOBEC3Q APOBEC3H, APOBEC4 (see, e.g., Yang et al., J Genet Genomics. 2017 Sep. 20; 44(9):423-437); activation-induced cytidine deaminase (AID), e.g., activation induced cytidine deaminase (AICDA), cytosine deaminase 1 (CDA1), and CDA2, and cytosine deaminase acting on tRNA (CDAT). The following Table 1 provides exemplary sequences; other sequences can also be used.

TABLE 1 Exemplary Deaminases GenBank Accession Nos. Deaminase Nucleic Acid Amino Acid hAID/AICDA NM_020661.3 isoform 1 NP_065712.1 variant 1 NM_020661.3 isoform 2 NP_065712.1 variant 2 APOBEC1 NM_001644.4 isoform a NP_001635.2 variant 1 NM_005889.3 isoform b NP_005880.2 variant 3 APOBEC2 NM_006789.3 NP_006780.1 APOBEC3A NM_145699.3 isoform a NP_663745.1 variant 1 NM_001270406.1 NP_001257335.1 isoform b variant 2 APOBEC3B NM_004900.4 isoform a NP_004891.4 variant 1 NM_001270411.1 NP_001257340.1 isoform b variant 2 APOBEC3C NM_014508.2 NP_055323.2 APOBEC3D/E NM_152426.3 NP_689639.2 APOBEC3F NM_145298.5 isoform a NP_660341.2 variant 1 NM_001006666.1 NP_001006667.1 isoform b variant 2 APOBEC3G NM_021822.3 (isoform a) NP_068594.1 (variant 1) APOBEC3H NM_001166003.2 NP_001159475.2 (variant SV-200) APOBEC4 NM_203454.2 NP_982279.1 CDA1* NM_127515.4 NP_179547.1 *from Saccharomyces cerevisiae S288C

The human AID (hAID), human APOBEC3 and mouse APOBEC3 enzymes (APO3A-hAPO3H, mAPO3) possess specificity for one, two or three additional nucleotides surrounding the target cytosine (Table 2)¹¹⁻¹⁶. The added specificity from 1 to 3 additional nucleotides would result in a 4- to 64-fold decreased probability of a non-target cytosine in a BE editing window being available for deamination in randomly distributed DNA. This would substantially enhance specificity of base editing enzymes within the genome at both on-target sites and nCas9 off-target sites with a high degree of similarity to the sgRNA, and would also contribute to limiting potential spurious sgRNA/nCas9-independent deamination throughout the genome by greatly reducing the number of total substrate sites in the genome. Importantly, the intrinsic sequence specificity of the hAPO3 and mAPO3 enzymes raises the possibility that these proteins could be engineered to alter or reassign their target sequence specificities. In this scenario, one could potentially engineer many different 2- and 3-bp recognizing deaminases to accommodate each target substrate sequence signature of interest.

TABLE 2 Cytidine deaminase domains described in this work and their substrate sequence preferences. Variant Nucleotide sequence preference hAID 5′-WRC rAPOBEC1* 5′-TC ≥ CC ≥ AC ≥ GC mAPOBEC3 5′-TYC hAPOBEC3A 5′-TCY hAPOBEC3B 5′-TCR ≥ TCT hAPOBEC3C 5′-WYC hAPOBEC3F 5′-TTC hAPOBEC3G 5′-CCC hAPOBEC3H 5′-TTCA ~ TTCT ~ TTCG > ACCCA > TGCA Nucleotide positions that are poorly specified or are permissive of two or more nucleotides are annotated according to IUPAC codes, where W = A or T, R = A or G, and Y = C or T.

Each endogenous hAPO3 and mAPO3 enzyme has an intrinsic 2-3 nt target substrate motif preference at which it can act. For instance, APO3A deaminates the bold cytosine in TC(A/G), while hAPO3G targets substrates of the form CCC. It would be advantageous to be able to modify the range of sequence motifs that are targetable by a given APO3 enzyme so as to increase the overall targeting range of this proposed class of substrate-specific APO3-containing BE reagents. In addition, engineering the substrate-specificity-determining residues of the APO3 enzymes will allow us to not only alter the preferred substrate site, but also improve the specificity of the APO3 enzyme for its preferred site relative to other closely matched di- or tri-nucleotide signatures that may exist abundantly across the genome. hAPO3 enzymes recognize their cognate sequence motifs through direct contacts formed between residues in two recognition loops with variable sequence composition termed recognition loop 1 (RL1) and Loop 7^(12-13, 17-22) (see Table 3, FIGS. 2A-2B, and FIGS. 3A-3B). Others have previously described substitution mutations made to these residues that relax or alter the specificity of these domains. For instance, mutating hAPO3G Loop 7 residues D316 and D317 to arginine changes the substrate specificity of the enzyme from CCC to CCC (wherein the bold C represents the substrate for deamination)²³. Mutating hAPO3F D311A increases targeting of TGC and TCC, where the unmodified enzyme prefers to target TTC¹⁵. Further, a chimeric APO3A protein in which the RL1 residues have been replaced by the RL1 residues of hAPO3G significantly relaxes specificity of the enzyme²⁴, and substituting hAID Loop 7 residues with those from hAPO3G or hAPO3F changes the target sequence profiles of the mutant hAID enzymes to resemble those of the donor enzymes²⁵. This work establishes that substitution mutations to residues in RL1 and Loop 7 are predicted to alter specificity of the APO3 proteins by altering the base-specific contacts made between the protein and the target ssDNA, but as yet there have been no reports of re-engineering APO3 substrate sequence specificity beyond these initial studies. Further, to our knowledge, no groups have described strategies to heighten the specificity of these domains for their target sequence motifs.

TABLE 3 Recognition Loop Residues Recognition Deaminase loop 1 residues Loop 7 residues hAID V18 - K22 A111 - P123 rAPOBEC1* D11 - R15 A117 - R126 mAPOBEC3* L33 - K37 S129 - E138 hAPOBEC3A G25 - R28 A127 - L135 hAPOBEC3C L24 - N28 A121 - C130 hAPOBEC3G E209 - R213 A312 - R320 hAPOBEC3H* K16 - Y23 S109 - P118 hAPOBEC3F L207 - Y211 A304 - D313 Each Recognition loop 1 and Loop 7 region represents a region of the enzyme either known to contribute to the sequence specificity of the enzyme or predicted to contribute to sequence specificity based on sequence alignment (see FIGS. 3A-3B) where structural information is lacking (*indicates which proteins lack sufficient structural information). All protein sequences acquired from uniprot.org. All positional information refers to positions within the full-length protein sequences as described below.

To this end, substitution mutations can be made to residues present in RL1 and/or Loop 7 (see Table 3 for residues corresponding to each APO3 or AID enzyme and FIGS. 2A-2B for an alignment of these proteins). To sample the greatest number of amino acid substitution combinations at positions in RL1 or Loop 7, we will randomize the amino acids in these loops and use a bacterial selection scheme to select for the library members with robust deamination activity on the desired sequence. In the case of Loop 7, residues believed to be most responsible for specifying the sequence motif will be individually mutated to residues that might be expected to form base-specific interactions with the desired sequence motif and the remaining positions in Loop 7 will be randomized. For example, APO3A residue D131 is known to form a hydrogen bond with 5′-TCY (wherein the bold T represents the base-specific contact). To alter the sequence specificity of APO3A from TCT>ACT, we will first design APO3A mutants bearing substitutions of D131 to N, Q, or R. Each of these APO3A variants will then be modified at positions 132-134 by cloning a synthesized degenerate oligonucleotide library made using an NNB, NNS or NNK reduced codon set (where N=A/C/G/T, B=C/G/T, S=C/G, and K=G/T).

The substitution mutations made to each of these residues include any of the 20 canonical amino acids in any combination with substitution mutations made to any of the other specified residue positions. Further, these mutations may be made in combination with truncating the enzymes in Table 3 to the minimal domain required for catalytic activity (catalytic domain; CD). See Exemplary Protein Sequences for examples of CDs derived from a subset of the enzymes described herein. These mutations may also be made in the presence or absence of all or any combination of 5 substitution mutations previously demonstrated to increase solubility and catalytic activity of hAPO3G (see Table 4 for a complete list of the homologous substitution mutations for each APO or AID enzyme described herein). Notably, the deaminase domains listed in Table 1 may have favorable intrinsic properties relative to rAPOBEC1 absent of any engineering we might do. As such, we describe herein the unmodified domains listed in Table 1 in addition to any engineered variants we may produce.

TABLE 4 Exemplary Substitution Mutations Deaminase Sub. 1 Sub. 2 Sub. 3 Sub. 4 Sub. 5 hAID — — F109K — — rAPOBEC1* L39K — Y115K — — mAPOBEC3* — — F127K — — hAPOBEC3A — — F125K — C171A hAPOBEC3G L234K C243A F310K C321A C356A hAPOBEC3H* — — F107K — — hAPOBEC3F — — F302K — — Each substitution mutation represents a residue of the enzyme either known to contribute to stability and solubility of the enzyme or predicted to contribute to sequence specificity based on sequence alignment to hAPOBEC3G where structural information is lacking (* indicates which proteins lack sufficient structural information). All positional information refers to the wild-type protein sequences acquired from uniprot.org. The exact position of these residues may change in engineered variants such as hAIDv. All positional information refers to positions within the full-length protein sequences as described below.

CRISPR-Cas Nucleases

The sequence specific deaminases described herein are fused to a Cas9 nickase. Although herein we refer to nCas9, in general any Cas9-like nickase could be used based on any ortholog of the Cpf1 protein (including the related Cpf1 enzyme class), unless specifically indicated. Table 5 provides an exemplary list.

TABLE 5 List of Exemplary Cas9 Orthologs Nickase UniProt Mutations/ Accession Catalytic Ortholog Number residues S. pyogenes Q99ZW2 D10A, E762A, Cas9 (SpCas9) H840A, N854A, N863A, D986A¹⁸ S. aureus J7RUA5 D10A and N58018¹⁹ Cas9 (SaCas9) S. thermophilus G3ECR1 D31A and N891A²⁰ Cas9 (St1Cas9) S. pasteurianus F5X275 D10, H599* Cas9 (SpaCas9) C. jejuni Q0P897 D8A, H559A²¹ Cas9 (CjCas9) F. novicida A0Q5Y3 D11, N995²² Cas9 (FnCas9) P. lavamentivorans A7HP89 D8, H601* Cas9 (PlCas9) C. lari G1UFN3 D7, H567* Cas9 (ClCas9) F. novicida A0Q7Q2 D917, E1006, Cpf1 (FnCpf1) D1255²³ M. bovoculi Sequence N/A** Cpf1 (MbCpf1) given at end A. sp. BV3L6 U2UMQ6 D908, 993E, (AsCpf1) Q1226, D1263²⁴ L. bacterium A0A182DWE3 D832A²⁵ N2006 (LbCpf1) These orthologs, and mutants and variants thereof as known in the art, can be used in any of the fusion proteins described herein. See, e.g., WO 2017/040348 (which describes variants of SaCas9 and SpCas 9 with increased specificity) and WO 2016/141224 (which describes variants of SaCas9 and SpCas 9 with altered PAM specificity).

The Cas9 nuclease from S. pyogenes (hereafter simply Cas9) can be guided via simple base pair complementarity between 17-20 nucleotides of an engineered guide RNA (gRNA), e.g., a single guide RNA or crRNA/tracrRNA pair, and the complementary strand of a target genomic DNA sequence of interest that lies next to a protospacer adjacent motif (PAM), e.g., a PAM matching the sequence NGG or NAG (Shen et al., Cell Res (2013); Dicarlo et al., Nucleic Acids Res (2013); Jiang et al., Nat Biotechnol 31, 233-239 (2013); Jinek et al., Elife 2, e00471 (2013); Hwang et al., Nat Biotechnol 31, 227-229 (2013); Cong et al., Science 339, 819-823 (2013); Mali et al., Science 339, 823-826 (2013c); Cho et al., Nat Biotechnol 31, 230-232 (2013); Jinek et al., Science 337, 816-821 (2012)). The engineered CRISPR from Prevotella and Francisella 1 (Cpf1) nuclease can also be used, e.g., as described in Zetsche et al., Cell 163, 759-771 (2015); Schunder et al., Int J Med Microbiol 303, 51-60 (2013); Makarova et al., Nat Rev Microbiol 13, 722-736 (2015); Fagerlund et al., Genome Biol 16, 251 (2015). Unlike SpCas9, Cpf1 requires only a single 42-nt crRNA, which has 23 nt at its 3′ end that are complementary to the protospacer of the target DNA sequence (Zetsche et al., 2015). Furthermore, whereas SpCas9 recognizes an NGG PAM sequence that is 3′ of the protospacer, AsCpf1 and LbCp1 recognize TTTN PAMs that are found 5′ of the protospacer (Id.).

In some embodiments, the present system utilizes a wild type or variant Cas9 protein from S. pyogenes or Staphylococcus aureus, or a wild type Cpf1 protein from Acidaminococcus sp. BV3L6 or Lachnospiraceae bacterium ND2006 either as encoded in bacteria or codon-optimized for expression in mammalian cells and/or modified in its PAM recognition specificity and/or its genome-wide specificity. A number of variants have been described; see, e.g., WO 2016/141224, PCT/US2016/049147, Kleinstiver et al., Nat Biotechnol. 2016 August; 34(8):869-74; Tsai and Joung, Nat Rev Genet. 2016 May; 17(5):300-12; Kleinstiver et al., Nature. 2016 Jan. 28; 529(7587):490-5; Shmakov et al., Mol Cell. 2015 Nov. 5; 60(3):385-97; Kleinstiver et al., Nat Biotechnol. 2015 December; 33(12):1293-1298; Dahlman et al., Nat Biotechnol. 2015 November; 33(11):1159-61; Kleinstiver et al., Nature. 2015 Jul. 23; 523(7561):481-5; Wyvekens et al., Hum Gene Ther. 2015 July; 26(7):425-31; Hwang et al., Methods Mol Biol. 2015; 1311:317-34; Osborn et al., Hum Gene Ther. 2015 February; 26(2):114-26; Konermann et al., Nature. 2015 Jan. 29; 517(7536):583-8; Fu et al., Methods Enzymol. 2014; 546:21-45; and Tsai et al., Nat Biotechnol. 2014 Jun.; 32(6):569-76, inter alia. The guide RNA is expressed or present in the cell together with the Cas9 or Cpf1. Either the guide RNA or the nuclease, or both, can be expressed transiently or stably in the cell or introduced as a purified protein or nucleic acid.

In some embodiments, the Cas9 also includes one of the following mutations, which reduce nuclease activity of the Cas9; e.g., for SpCas9, mutations at D10A or H840A (which creates a single-strand nickase).

In some embodiments, the SpCas9 variants also include mutations at one of the following amino acid positions, which destroy the nuclease activity of the Cas9: D10, E762, D839, H983, or D986 and H840 or N863, e.g., D10A/D10N and H840A/H840N/H840Y, to render the nuclease portion of the protein catalytically inactive; substitutions at these positions could be alanine (as they are in Nishimasu al., Cell 156, 935-949 (2014)), or other residues, e.g., glutamine, asparagine, tyrosine, serine, or aspartate, e.g., E762Q, H983N, H983Y, D986N, N863D, N863S, or N863H (see WO 2014/152432).

In some embodiments, the Cas9 is fused to one or more Uracil glycosylase inhibitor (UGI) sequences; an exemplary UGI sequence is as follows: TNLSDIIEKETGKQLVIQESILMLPEEVEEVIGNKPESDILVHTAYDESTDENVM LLTSDAPEYKPWALVIQDSNGENKIKML (SEQ ID NO:45). The UGI can be N terminal, C terminal, or absent (and optionally expressed in trans, e.g., separately, or provided or administered separately).

Methods of Use

The present compositions and methods can be used to enhance genome-wide specificity by engineered APO3A deaminases. For example, APO3A N57G employed in the BE3 architecture has increased genome-wide specificity at off target sites determined by the identity of the spacer sequence of the guide RNA when delivered by transient plasmid transfection. In addition, the methods can include delivery of APO3A BE3 with any of the mutations in Table 7, e.g., using RNP or mRNA transfection to limit genome-wide off target mutagenesis of base editor reagents. Additionally, the APO3A BE3 with any of the mutations in Table 7 (by themselves or together) can be delivered using transient plasmid transfection, RNP, or mRNA where the ssDNA nicking or catalytically-inactive Cas9 (nCas9) is any engineered SpCas9 protein⁴⁶ that recognizes an orthogonal PAM sequence to the SpCas9 NGG PAM in order to limit off target mutagenesis by the fusion protein. Additionally, APO3A can also be used with any of the mutations in Table 7 fused to S. aureus Cas9 or the engineered S. aureus Cas9 that recognizes an orthogonal PAM sequence⁴⁷.

Variants

In some embodiments, the components of the fusion proteins are at least 80%, e.g., at least 85%, 90%, 95%, 97%, or 99% identical to the amino acid sequence of a exemplary sequence (e.g., as provided herein), e.g., have differences at up to 5%, 10%, 15%, or 20% of the residues of the exemplary sequence replaced, e.g., with conservative mutations, in addition to the mutations described herein. In preferred embodiments, the variant retains desired activity of the parent, e.g., the nickase activity, and/or the ability to interact with a guide RNA and/or target DNA).

To determine the percent identity of two nucleic acid sequences, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in one or both of a first and a second amino acid or nucleic acid sequence for optimal alignment and non-homologous sequences can be disregarded for comparison purposes). The length of a reference sequence aligned for comparison purposes is at least 80% of the length of the reference sequence, and in some embodiments is at least 90% or 100%. The nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same nucleotide as the corresponding position in the second sequence, then the molecules are identical at that position (as used herein nucleic acid “identity” is equivalent to nucleic acid “homology”). The percent identity between the two sequences is a function of the number of identical positions shared by the sequences, taking into account the number of gaps, and the length of each gap, which need to be introduced for optimal alignment of the two sequences. Percent identity between two polypeptides or nucleic acid sequences is determined in various ways that are within the skill in the art, for instance, using publicly available computer software such as Smith Waterman Alignment (Smith, T. F. and M. S. Waterman (1981) J Mol Biol 147:195-7); “BestFit” (Smith and Waterman, Advances in Applied Mathematics, 482-489 (1981)) as incorporated into GeneMatcher Plus™, Schwarz and Dayhof (1979) Atlas of Protein Sequence and Structure, Dayhof, M. O., Ed, pp 353-358; BLAST program (Basic Local Alignment Search Tool; (Altschul, S. F., W. Gish, et al. (1990) J Mol Biol 215: 403-10), BLAST-2, BLAST-P, BLAST-N, BLAST-X, WU-BLAST-2, ALIGN, ALIGN-2, CLUSTAL, or Megalign (DNASTAR) software. In addition, those skilled in the art can determine appropriate parameters for measuring alignment, including any algorithms needed to achieve maximal alignment over the length of the sequences being compared. In general, for proteins or nucleic acids, the length of comparison can be any length, up to and including full length (e.g., 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or 100%). For purposes of the present compositions and methods, at least 80% of the full length of the sequence is aligned.

For purposes of the present disclosure, the comparison of sequences and determination of percent identity between two sequences can be accomplished using a Blossum 62 scoring matrix with a gap penalty of 12, a gap extend penalty of 4, and a frameshift gap penalty of 5.

Conservative substitutions typically include substitutions within the following groups: glycine, alanine; valine, isoleucine, leucine; aspartic acid, glutamic acid, asparagine, glutamine; serine, threonine; lysine, arginine; and phenylalanine, tyrosine.

Also provided herein are isolated nucleic acids encoding the split deaminase fusion proteins, vectors comprising the isolated nucleic acids, optionally operably linked to one or more regulatory domains for expressing the variant proteins, and host cells, e.g., mammalian host cells, comprising the nucleic acids, and optionally expressing the variant proteins. In some embodiments, the host cells are stem cells, e.g., hematopoietic stem cells.

The split deaminase fusion proteins described herein can be used for altering the genome of a cell. The methods generally include expressing or contacting the split deaminase fusion proteins in the cells; in versions using one or two Cas9s, the methods include using a guide RNA having a region complementary to a selected portion of the genome of the cell. Methods for selectively altering the genome of a cell are known in the art, see, e.g., U.S. Pat. No. 8,993,233; US 20140186958; U.S. Pat. No. 9,023,649; WO/2014/099744; WO 2014/089290; WO2014/144592; WO144288; WO2014/204578; WO2014/152432; WO2115/099850; U.S. Pat. No. 8,697,359; US20160024529; US20160024524; US20160024523; US20160024510; US20160017366; US20160017301; US20150376652; US20150356239; US20150315576; US20150291965; US20150252358; US20150247150; US20150232883; US20150232882; US20150203872; US20150191744; US20150184139; US20150176064; US20150167000; US20150166969; US20150159175; US20150159174; US20150093473; US20150079681; US20150067922; US20150056629; US20150044772; US20150024500; US20150024499; US20150020223; US20140356867; US20140295557; US20140273235; US20140273226; US20140273037; US20140189896; US20140113376; US20140093941; US20130330778; US20130288251; US20120088676; US20110300538; US20110236530; US20110217739; US20110002889; US20100076057; US20110189776; US20110223638; US20130130248; US20150050699; US20150071899; US20150050699; US20150045546; US20150031134; US20150024500; US20140377868; US20140357530; US20140349400; US20140335620; US20140335063; US20140315985; US20140310830; US20140310828; US20140309487; US20140304853; US20140298547; US20140295556; US20140294773; US20140287938; US20140273234; US20140273232; US20140273231; US20140273230; US20140271987; US20140256046; US20140248702; US20140242702; US20140242700; US20140242699; US20140242664; US20140234972; US20140227787; US20140212869; US20140201857; US20140199767; US20140189896; US20140186958; US20140186919; US20140186843; US20140179770; US20140179006; US20140170753; WO/2008/108989; WO/2010/054108; WO/2012/164565; WO/2013/098244; WO/2013/176772; US 20150071899; Makarova et al., “Evolution and classification of the CRISPR-Cas systems” 9(6) Nature Reviews Microbiology 467-477 (1-23) (June 2011); Wiedenheft et al., “RNA-guided genetic silencing systems in bacteria and archaea” 482 Nature 331-338 (Feb. 16, 2012); Gasiunas et al., “Cas9-crRNA ribonucleoprotein complex mediates specific DNA cleavage for adaptive immunity in bacteria” 109(39) Proceedings of the National Academy of Sciences USA E2579-E2586 (Sep. 4, 2012); Jinek et al., “A Programmable Dual-RNA-Guided DNA Endonuclease in Adaptive Bacterial Immunity” 337 Science 816-821 (Aug. 17, 2012); Carroll, “A CRISPR Approach to Gene Targeting” 20(9) Molecular Therapy 1658-1660 (September 2012); U.S. Appl. No. 61/652,086, filed May 25, 2012; Al-Attar et al., Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs): The Hallmark of an Ingenious Antiviral Defense Mechanism in Prokaryotes, Biol Chem. (2011) vol. 392, Issue 4, pp. 277-289; Hale et al., Essential Features and Rational Design of CRISPR RNAs That Function With the Cas RAMP Module Complex to Cleave RNAs, Molecular Cell, (2012) vol. 45, Issue 3, 292-302.

In some embodiments, the fusion proteins include a linker between the DNA binding domain (e.g., ZFN, TALE, or nCas9) and the BE domains. Linkers that can be used in these fusion proteins (or between fusion proteins in a concatenated structure) can include any sequence that does not interfere with the function of the fusion proteins. In preferred embodiments, the linkers are short, e.g., 2-20 amino acids, and are typically flexible (i.e., comprising amino acids with a high degree of freedom such as glycine, alanine, and serine). In some embodiments, the linker comprises one or more units consisting of GGGS (SEQ ID NO:46) or GGGGS (SEQ ID NO:47), e.g., two, three, four, or more repeats of the GGGS (SEQ ID NO:47) or GGGGS (SEQ ID NO:46) unit. Other linker sequences can also be used.

In some embodiments, the split deaminase fusion protein includes a cell-penetrating peptide sequence that facilitates delivery to the intracellular space, e.g., HIV-derived TAT peptide, penetratins, transportans, or hCT derived cell-penetrating peptides, see, e.g., Caron et al., (2001) Mol Ther. 3(3):310-8; Langel, Cell-Penetrating Peptides: Processes and Applications (CRC Press, Boca Raton Fla. 2002); El-Andaloussi et al., (2005) Curr Pharm Des. 11(28):3597-611; and Deshayes et al., (2005) Cell Mol Life Sci. 62(16):1839-49.

Cell penetrating peptides (CPPs) are short peptides that facilitate the movement of a wide range of biomolecules across the cell membrane into the cytoplasm or other organelles, e.g. the mitochondria and the nucleus. Examples of molecules that can be delivered by CPPs include therapeutic drugs, plasmid DNA, oligonucleotides, siRNA, peptide-nucleic acid (PNA), proteins, peptides, nanoparticles, and liposomes. CPPs are generally 30 amino acids or less, are derived from naturally or non-naturally occurring protein or chimeric sequences, and contain either a high relative abundance of positively charged amino acids, e.g. lysine or arginine, or an alternating pattern of polar and non-polar amino acids. CPPs that are commonly used in the art include Tat (Frankel et al., (1988) Cell. 55:1189-1193, Vives et al., (1997) J. Biol. Chem. 272:16010-16017), penetratin (Derossi et al., (1994) J. Biol. Chem. 269:10444-10450), polyarginine peptide sequences (Wender et al., (2000) Proc. Natl. Acad. Sci. USA 97:13003-13008, Futaki et al., (2001) J. Biol. Chem. 276:5836-5840), and transportan (Pooga et al., (1998) Nat. Biotechnol. 16:857-861).

CPPs can be linked with their cargo through covalent or non-covalent strategies. Methods for covalently joining a CPP and its cargo are known in the art, e.g. chemical cross-linking (Stetsenko et al., (2000) J. Org. Chem. 65:4900-4909, Gait et al. (2003) Cell. Mol. Life. Sci. 60:844-853) or cloning a fusion protein (Nagahara et al., (1998) Nat. Med. 4:1449-1453). Non-covalent coupling between the cargo and short amphipathic CPPs comprising polar and non-polar domains is established through electrostatic and hydrophobic interactions.

CPPs have been utilized in the art to deliver potentially therapeutic biomolecules into cells. Examples include cyclosporine linked to polyarginine for immunosuppression (Rothbard et al., (2000) Nature Medicine 6(11):1253-1257), siRNA against cyclin B1 linked to a CPP called MPG for inhibiting tumorigenesis (Crombez et al., (2007) Biochem Soc. Trans. 35:44-46), tumor suppressor p53 peptides linked to CPPs to reduce cancer cell growth (Takenobu et al., (2002) Mol. Cancer Ther. 1(12):1043-1049, Snyder et al., (2004) PLoS Biol. 2:E36), and dominant negative forms of Ras or phosphoinositol 3 kinase (PI3K) fused to Tat to treat asthma (Myou et al., (2003) J. Immunol. 171:4399-4405).

CPPs have been utilized in the art to transport contrast agents into cells for imaging and biosensing applications. For example, green fluorescent protein (GFP) attached to Tat has been used to label cancer cells (Shokolenko et al., (2005) DNA Repair 4(4):511-518). Tat conjugated to quantum dots have been used to successfully cross the blood-brain barrier for visualization of the rat brain (Santra et al., (2005) Chem. Commun. 3144-3146). CPPs have also been combined with magnetic resonance imaging techniques for cell imaging (Liu et al., (2006) Biochem. and Biophys. Res. Comm. 347(1):133-140). See also Ramsey and Flynn, Pharmacol Ther. 2015 Jul. 22. pii: 50163-7258(15)00141-2.

Alternatively or in addition, the split deaminase fusion proteins can include a nuclear localization sequence, e.g., SV40 large T antigen NLS (PKKKRRV (SEQ ID NO:48)) and nucleoplasmin NLS (KRPAATKKAGQAKKKK (SEQ ID NO:49)). Other NLSs are known in the art; see, e.g., Cokol et al., EMBO Rep. 2000 Nov. 15; 1(5): 411-415; Freitas and Cunha, Curr Genomics. 2009 December; 10(8): 550-557.

In some embodiments, the split deaminase fusion proteins include a moiety that has a high affinity for a ligand, for example GST, FLAG or hexahistidine sequences. Such affinity tags can facilitate the purification of recombinant split deaminase fusion proteins.

For methods in which the split deaminase fusion proteins are delivered to cells, the proteins can be produced using any method known in the art, e.g., by in vitro translation, or expression in a suitable host cell from nucleic acid encoding the split deaminase fusion protein; a number of methods are known in the art for producing proteins. For example, the proteins can be produced in and purified from yeast, E. coli, insect cell lines, plants, transgenic animals, or cultured mammalian cells; see, e.g., Palomares et al., “Production of Recombinant Proteins: Challenges and Solutions,” Methods Mol Biol. 2004; 267:15-52. In addition, the split deaminase fusion proteins can be linked to a moiety that facilitates transfer into a cell, e.g., a lipid nanoparticle, optionally with a linker that is cleaved once the protein is inside the cell. See, e.g., LaFountaine et al., Int J Pharm. 2015 Aug. 13; 494(1):180-194.

Expression Systems

To use the split deaminase fusion proteins described herein, it may be desirable to express them from a nucleic acid that encodes them. This can be performed in a variety of ways. For example, the nucleic acid encoding the split deaminase fusion can be cloned into an intermediate vector for transformation into prokaryotic or eukaryotic cells for replication and/or expression. Intermediate vectors are typically prokaryote vectors, e.g., plasmids, or shuttle vectors, or insect vectors, for storage or manipulation of the nucleic acid encoding the split deaminase fusion for production of the split deaminase fusion protein. The nucleic acid encoding the split deaminase fusion protein can also be cloned into an expression vector, for administration to a plant cell, animal cell, preferably a mammalian cell or a human cell, fungal cell, bacterial cell, or protozoan cell.

To obtain expression, a sequence encoding a split deaminase fusion protein is typically subcloned into an expression vector that contains a promoter to direct transcription. Suitable bacterial and eukaryotic promoters are well known in the art and described, e.g., in Sambrook et al., Molecular Cloning, A Laboratory Manual (3d ed. 2001); Kriegler, Gene Transfer and Expression: A Laboratory Manual (1990); and Current Protocols in Molecular Biology (Ausubel et al., eds., 2010). Bacterial expression systems for expressing the engineered protein are available in, e.g., E. coli, Bacillus sp., and Salmonella (Palva et al., 1983, Gene 22:229-235). Kits for such expression systems are commercially available. Eukaryotic expression systems for mammalian cells, yeast, and insect cells are well known in the art and are also commercially available.

The promoter used to direct expression of a nucleic acid depends on the particular application. For example, a strong constitutive promoter is typically used for expression and purification of fusion proteins. In contrast, when the split deaminase fusion protein is to be administered in vivo for gene regulation, either a constitutive or an inducible promoter can be used, depending on the particular use of the split deaminase fusion protein. In addition, a preferred promoter for administration of the split deaminase fusion protein can be a weak promoter, such as HSV TK or a promoter having similar activity. The promoter can also include elements that are responsive to transactivation, e.g., hypoxia response elements, Gal4 response elements, lac repressor response element, and small molecule control systems such as tetracycline-regulated systems and the RU-486 system (see, e.g., Gossen & Bujard, 1992, Proc. Natl. Acad. Sci. USA, 89:5547; Oligino et al., 1998, Gene Ther., 5:491-496; Wang et al., 1997, Gene Ther., 4:432-441; Neering et al., 1996, Blood, 88:1147-55; and Rendahl et al., 1998, Nat. Biotechnol., 16:757-761).

In addition to the promoter, the expression vector typically contains a transcription unit or expression cassette that contains all the additional elements required for the expression of the nucleic acid in host cells, either prokaryotic or eukaryotic. A typical expression cassette thus contains a promoter operably linked, e.g., to the nucleic acid sequence encoding the split deaminase fusion protein, and any signals required, e.g., for efficient polyadenylation of the transcript, transcriptional termination, ribosome binding sites, or translation termination. Additional elements of the cassette may include, e.g., enhancers, and heterologous spliced intronic signals.

The particular expression vector used to transport the genetic information into the cell is selected with regard to the intended use of the split deaminase fusion protein, e.g., expression in plants, animals, bacteria, fungus, protozoa, etc. Standard bacterial expression vectors include plasmids such as pBR322 based plasmids, pSKF, pET23D, and commercially available tag-fusion expression systems such as GST and LacZ.

Expression vectors containing regulatory elements from eukaryotic viruses are often used in eukaryotic expression vectors, e.g., SV40 vectors, papilloma virus vectors, and vectors derived from Epstein-Barr virus. Other exemplary eukaryotic vectors include pMSG pAV009/A+, pMTO10/A+, pMAMneo-5, baculovirus pDSVE, and any other vector allowing expression of proteins under the direction of the SV40 early promoter, SV40 late promoter, metallothionein promoter, murine mammary tumor virus promoter, Rous sarcoma virus promoter, polyhedrin promoter, or other promoters shown effective for expression in eukaryotic cells.

The vectors for expressing the split deaminase fusion protein can include RNA Pol III promoters to drive expression of the guide RNAs, e.g., the H1, U6 or 7SK promoters. These human promoters allow for expression of split deaminase fusion protein in mammalian cells following plasmid transfection.

Some expression systems have markers for selection of stably transfected cell lines such as thymidine kinase, hygromycin B phosphotransferase, and dihydrofolate reductase. High yield expression systems are also suitable, such as using a baculovirus vector in insect cells, with the gRNA encoding sequence under the direction of the polyhedrin promoter or other strong baculovirus promoters.

The elements that are typically included in expression vectors also include a replicon that functions in E. coli, a gene encoding antibiotic resistance to permit selection of bacteria that harbor recombinant plasmids, and unique restriction sites in nonessential regions of the plasmid to allow insertion of recombinant sequences.

Standard transfection methods are used to produce bacterial, mammalian, yeast or insect cell lines that express large quantities of protein, which are then purified using standard techniques (see, e.g., Colley et al., 1989, J. Biol. Chem., 264:17619-22; Guide to Protein Purification, in Methods in Enzymology, vol. 182 (Deutscher, ed., 1990)). Transformation of eukaryotic and prokaryotic cells are performed according to standard techniques (see, e.g., Morrison, 1977, J. Bacteriol. 132:349-351; Clark-Curtiss & Curtiss, Methods in Enzymology 101:347-362 (Wu et al., eds, 1983).

Any of the known procedures for introducing foreign nucleotide sequences into host cells may be used. These include the use of calcium phosphate transfection, polybrene, protoplast fusion, electroporation, nucleofection, liposomes, microinjection, naked DNA, plasmid vectors, viral vectors, both episomal and integrative, and any of the other well-known methods for introducing cloned genomic DNA, cDNA, synthetic DNA or other foreign genetic material into a host cell (see, e.g., Sambrook et al., supra). It is only necessary that the particular genetic engineering procedure used be capable of successfully introducing at least one gene into the host cell capable of expressing the split deaminase fusion protein.

Alternatively, the methods can include delivering the split deaminase fusion protein and guide RNA together, e.g., as a complex. For example, the split deaminase fusion protein and gRNA can be can be overexpressed in a host cell and purified, then complexed with the guide RNA (e.g., in a test tube) to form a ribonucleoprotein (RNP), and delivered to cells. In some embodiments, the split deaminase fusion protein can be expressed in and purified from bacteria through the use of bacterial expression plasmids. For example, His-tagged split deaminase fusion protein can be expressed in bacterial cells and then purified using nickel affinity chromatography. The use of RNPs circumvents the necessity of delivering plasmid DNAs encoding the nuclease or the guide, or encoding the nuclease as an mRNA. RNP delivery may also improve specificity, presumably because the half-life of the RNP is shorter and there's no persistent expression of the nuclease and guide (as you'd get from a plasmid). The RNPs can be delivered to the cells in vivo or in vitro, e.g., using lipid-mediated transfection or electroporation. See, e.g., Liang et al. “Rapid and highly efficient mammalian cell engineering via Cas9 protein transfection.” Journal of biotechnology 208 (2015): 44-53; Zuris, John A., et al. “Cationic lipid-mediated delivery of proteins enables efficient protein-based genome editing in vitro and in vivo.” Nature biotechnology 33.1 (2015): 73-80; Kim et al. “Highly efficient RNA-guided genome editing in human cells via delivery of purified Cas9 ribonucleoproteins.” Genome research 24.6 (2014): 1012-1019.

The present invention also includes the vectors and cells comprising the vectors, as well as kits comprising the proteins and nucleic acids described herein, e.g., for use in a method described herein.

Treating Beta-Thalassemia

Beta-thalassemias are a group of hereditary disorders characterized by a genetic deficiency in the synthesis of beta-globin chains. Thalassemia major, in which the affected individual is homozygous for the mutation, is associated with severe anemia requiring transfusion. Thalassemia minor is the least severe and does not typically require treatment, while those with levels of severity between thalassemia minor and thalassemia major are said to have thalassemia intermedia. See, e.g., Cao and Galanello, Genetics in Medicine 12, 61-76 (2010);

The hAPOBEC3A mutants, e.g., N57G or N57A or N57Q or K60A or K60D or Y130F, as a fusion protein in the BE3 architecture as described herein can be used as a therapy in subjects, e.g., with the beta-thalassemia mutation HBB −28 (A>G) that is common in some east Asian populations (see Liang et al., Protein Cell. 2017 November; 8(11):811-822). Methods for identifying subjects with this mutation are known in the art; see, e.g., Saetung et al., Southeast Asian J Trop Med Public Health. 2013 November; 44(6):1055-64; Liu et al., Hemoglobin. 2015; 39(1):18-23; Doro et al., Hemoglobin. 2017 March; 41(2):96-99; Zhang et al., BMJ Open. 2017 Jan. 31; 7(1):e013367. The methods can include mobilizing and then extracting CD34+ hematopoietic stem and progenitor cells (HSPCs)(see, e.g., Bonig and Papayannopoulou, Methods Mol Biol. 2012; 904: 1-14; Jin, et al., BioMed Research International, vol. 2014, Article ID 435215, 9 pages, 2014). The HSPCs are then modified ex vivo (outside of the subject's body) by introducing mRNA encoding the base editor protein or by using purified base editor protein+guide (e.g., an RNP). The cells can be maintained in culture to allow for proliferation, e.g., for a few days, before infusing the modified cells back into the subject. The subject can also be myeloablated before infusion to ensure that the modified cells engraft well (see Sullivan, Keith M., et al., New England Journal of Medicine 378.1 (2018). 35-47.). The modified stem cells are then allowed to engraft in the subject's bone marrow and produce beta-thalassemia-free red blood cells.

As described herein, we investigated whether APO3A N57G BE3 was able to efficiently induce single nucleotide editing at the β-thalassemia causing allele HBB −28 (A>G) that is common in some east Asian populations. We first created a HEK293T model cell line bearing a singly-integrated 200 bp fragment of the disease-causing HBB −28 (A>G) promoter allele and tested whether APO3A N57G BE3 was able to more efficiently induce single nucleotide editing at the −28 (A>G) position (editing on the antisense strand, to affect the sense strand) relative to BE3 or the YE BE3 derivatives. As expected, APO3A N57G BE3 induced significantly fewer editing events at the HBB −25 bystander motif while retaining high editing activity at the −28 (A>G) cognate motif. Editing with BE3 produced 0.57% perfectly corrected alleles. Deamination of the HBB −25 cytidine in the editing window by BE3 produces the causal β-thalassemia mutations HBB −25 (G>T) and (G>C) mutations in 11.5% and 1.8% of alleles, respectively. The product −25 (G>A), present in 15.2% of total alleles after editing with BE3, may also produce an independent β-thalassemia phenotype, but this has yet to be clinically confirmed. Conversely, editing at the HBB −28 (A>G) site with APO3A N57G BE3 produced 22.5% perfectly corrected alleles, 40-fold more than BE3, and a total editing rate at the −25 position of 3.96%.

EXAMPLES

The invention is further described in the following examples, which do not limit the scope of the invention described in the claims.

Materials and Methods

Plasmids and Oligonucleotides

BE expression plasmids containing amino acid substitutions were generated by PCR and standard molecular cloning methods. gRNA expression plasmids were constructed by ligating annealed oligonucleotide duplexes into MLM3636 cut with BsmBI. All gRNAs except those targeting the HBB −28 (A>G) and CTNNB1 sites were designed to target sites containing a 5′ guanine nucleotide.

Human Cell Culture and Transfection

2OS.EGFP cells containing a single stably integrated copy of the EGFP-PEST reporter gene and HEK293T cells were cultured in DMEM supplemented with 10% heat-inactivated fetal bovine serum, 2 mM GlutaMax, penicillin and streptomycin at 37° C. with 5% CO₂. The media for U2OS.EGFP cells was supplemented with 400 μg ml⁻¹ Geneticin. Cell line identity was validated by STR profiling (ATCC), and cells were tested regularly for mycoplasma contamination. U2OS.EGFP cells were transfected with 750 ng of plasmid expressing BE and 250 ng of plasmid expressing sgRNA according to the manufacturer's recommendations using the DN-100 program and SE cell line kit on a Lonza 4-D Nucleofector. For HEK293T transfections, 75,000 cells were seeded in 24-well plates and 18 hours later were transfected with 600 ng of plasmid expressing BE and 200 ng of plasmid expressing sgRNA using TransIT-293 (Mirus) according to the manufacturer's recommendations. For all targeted amplicon sequencing and GUIDE-seq experiments, genomic DNA was extracted 72 h post-transfection. Cells were lysed in lysis buffer containing 100 mM Tris-HCl pH 8.0, 150 mM NaCl, 5 mM EDTA, and 0.05% SDS and incubated overnight at 55C in an incubator shaking at 250 rpm. Genomic DNA was extracted from lysed cells using carboxyl-modified Sera-Mag Magnetic Speed-beads resuspended in 2.5 M NaCl and 18% PEG-6000 (magnetic beads).

The HEK293T.HBB cell line was constructed by cloning a 200 base pair fragment of the HBB promoter upstream of an EF1a promoter driving expression of the puromycin resistance gene in a lentiviral vector. The HBB −28 (A>G) mutation was inserted by PCR and standard molecular cloning methods. The lentiviral vector was transfected into 293FS cells and media containing viral particles was harvested after 72 hours. Media containing viral particles was serially diluted and added to 10 cm plates with approximately 10 million HEK293T cells. After 48 hours, media was supplemented with 2.5 μg ml¹ puromycin and cells were harvested from the 10 cm plate with the fewest surviving colonies to ensure single copy integration.

Off-Target Site Selection and Amplicon Design

Two of the sites characterized here, EMX1 site 1 and FANCF, were previously characterized by modified Digenome-seq, an unbiased approach to discover BE3-specific off-target sites. All off-targets discovered by modified Digenome-seq were investigated, and these sites represent the most comprehensive off-target characterization because they were discovered de novo using BE3. The VEGFA site 2 target is a promiscuous, homopolymeric gRNA that was previously characterized by GUIDE-seq. Because the VEGFA site 2 gRNA has over one hundred nuclease off-target sites, we selected the 20 off-target sites with the highest number of GUIDE-seq reads that also reside in loci for which we were able to design unique PCR amplification primers for characterization here. The CTNNB1 and HBB −28 (A>G) gRNAs had not been previously characterized with respect to BE or nuclease off-target sites. We performed GUIDE-seq as previously described¹⁷ using these gRNAs to determine the SpCas9 nuclease off-target sites, and used Cas-OFFinder to predict all of the potential off-target sites with one RNA bulge and one mismatch. (GUIDE-seq and Cas-OFFinder analyses were performed using the hg38 reference genome.) This class of off-targets is more prevalent in BE3 relative to nucleases¹⁶, and thus sites that we were unlikely to discover by GUIDE-seq. Primers were designed to amplify all off-target sites such that potential edited cytidines were within the first 100 base pairs of Illumina HTS reads. A total of six primer pairs encompassing EVIX1 site 1, VEGFA site 2 and CTNNB1 site 1 off-target sites did not amplify their intended amplicon and were thus excluded from further analysis.

Statistical Testing

All statistical testing was performed using two-tailed Student's t-test according to the method of Benjamini, Krieger, and Yekutieli without assuming equal variances between samples.

Targeted Amplicon Sequencing

On- and off-target sites were amplified from ˜100 ng genomic DNA from three biological replicates for each condition. PCR amplification was performed with Phusion High Fidelity DNA Polymerase (NEB). 50 μl PCR reactions were purified with 1× volume magnetic beads. Amplification fidelity was verified by capillary electrophoresis on a Qiaxcel instrument. Amplicons with orthogonal sequences were pooled for each triplicate transfection and Illumina flow cell-compatible adapters were added using the NEBNext Ultra II DNA Library Prep kit according to manufacturer instructions. Illumina i5 and i7 indices were added by an additional 10 cycles of PCR with Q5 High Fidelity DNA Polymerase using primers from NEBNext Multiplex Oligos for Illumina (Dual Index Primers Set 1) and purified using 0.7× volume magnetic beads. Final amplicon libraries containing Illumina-compatible adapters and indices were quantified by droplet digital PCR and sequenced with 150 bp paired end reads on an Illumina MiSeq instrument. Sequencing reads were de-multiplexed by MiSeq Reporter then analyzed for base frequency at each position by a modified version of CRISPResso²⁸. Indels were quantified in a 10 base pair window surrounding the expected cut site for each sgRNA.

Expression of HBB −28 (A>G) gRNAs

In order to use eA3A BEs with the HF1 or Hypa mutations that decrease genome-wide off-target editing, it was necessary to use 20 nucleotides of spacer sequence in the gRNA with no mismatches between the spacer and target site. We expressed the HBB −28 (A>G) gRNA from a plasmid using the U6 promoter, which preferentially initiates transcription at a guanine nucleotide at the +1 position. To preserve perfect matching between the spacer and target site, we appended a self-cleaving 5′ hammerhead ribozyme that is able to remove the mismatched guanine at the 5′ of the spacer.

Exemplary protein sequences rAPOBEC1-XTEN L8-nCas9-UGI-SV40 NLS MSSETGPVAVDPTLRRRIEPHEFEVFFDPRELRKETCLLYEINWGGRHSIW RHTSQNTNKHVEVNFIEKFTTERYFCPNTRCSITWFLSWSPCGECSRAITEFLSRYP HVTLFIYIARLYHHADPRNRQGLRDLISSGVTIQIMTEQESGYCWRNFVNYSPSNEA HWPRYPHLVVVRLYVLELYCIILGLPPCLNILRRKQPQLTFFTIALQSCHYQRLPPHIL WATGLKSGSETPGTSESATPESDKKYSIGLAIGTNSVGWAVITDEYKVPSKKFKVLG NTDRHSIKKNLIGALLFDSGETAEATRLKRTARRRYTRRKNRICYLQEIFSNEMAKVD DSFFHRLEESFLVEEDKKHERHPIFGNIVDEVAYHEKYPTIYHLRKKLVDSTDKADLR LIYLALAHMIKFRGHFLIEGDLNPDNSDVDKLFIQLVQTYNQLFEENPINASGVDAKAI LSARLSKSRRLENLIAQLPGEKKNGLFGNLIALSLGLTPNFKSNFDLAEDAKLQLSKD TYDDDLDNLLAQIGDQYADLFLAAKNLSDAILLSDILRVNTEITKAPLSASMIKRYDEH HQDLTLLKALVRQQLPEKYKEIFFDQSKNGYAGYIDGGASQEEFYKFIKPILEKMDG TEELLVKLNREDLLRKQRTFDNGSIPHQIHLGELHAILRRQEDFYPFLKDNREKIEKIL TFRIPYYVGPLARGNSRFAWMTRKSEETITPWNFEEVVDKGASAQSFIERMTNFDK NLPNEKVLPKHSLLYEYFTVYNELTKVKYVTEGMRKPAFLSGEQKKAIVDLLFKTNR KVTVKQLKEDYFKKIECFDSVEISGVEDRFNASLGTYHDLLKIIKDKDFLDNEENEDIL EDIVLTLTLFEDREMIEERLKTYAHLFDDKVMKQLKRRRYTGWGRLSRKLINGIRDK QSGKTILDFLKSDGFANRNFMQLIHDDSLTFKEDIQKAQVSGQGDSLHEHIANLAGS PAIKKGILQTVKWDELVKVMGRHKPENIVIEMARENQTTQKGQKNSRERMKRIEEG IKELGSQILKEHPVENTQLQNEKLYLYYLQNGRDMYVDQELDINRLSDYDVDHIVPQ SFLKDDSIDNKVLTRSDKNRGKSDNVPSEEVVKKMKNYWRQLLNAKLITQRKFDNL TKAERGGLSELDKAGFIKRQLVETRQITKHVAQILDSRMNTKYDENDKLIREVKVITL KSKLVSDFRKDFQFYKVREINNYHHAHDAYLNAVVGTALIKKYPKLESEFVYGDYKV YDVRKMIAKSEQEIGKATAKYFFYSNIMNFFKTEITLANGEIRKRPLIETNGETGEIVW DKGRDFATVRKVLSMPQVNIVKKTEVQTGGFSKESILPKRNSDKLIARKKDWDPKK YGGFDSPTVAYSVLVVAKVEKGKSKKLKSVKELLGITIMERSSFEKNPIDFLEAKGYK EVKKDLIIKLPKYSLFELENGRKRMLASAGELQKGNELALPSKYVNFLYLASHYEKLK GSPEDNEQKQLFVEQHKHYLDEIIEQISEFSKRVILADANLDKVLSAYNKHRDKPIRE QAENIIHLFTLTNLGAPAAFKYFDTTIDRKRYTSTKEVLDATLIHQSITGLYETRIDLSQ LGGDSGGSTNLSDIIEKETGKQLVIQESILMLPEEVEEVIGNKPESDILVHTAYDESTD ENVMLLTSDAPEYKPWALVIQDSNGENKIKMLSGGSPKKKRKV (SEQ ID NO: 50) Uracil glycosylase inhibitor (UGI) TNLSDIIEKETGKQLVIQESILMLPEEVEEVIGNKPESDILVHTAYDEST DENVMLLTSDAPEYKPWALVIQDSNGENKIKML (SEQ ID NO: 45) hAID MDSLLMNRRKFLYQFKNVRWAKGRRETYLCYVVKRRDSATSFSLDFGYLR NKNGCHVELLFLRYISDWDLDPGRCYRVTWFTSWSPCYDCARHVADFLRGNPNLS LRIFTARLYFCEDRKAEPEGLRRLHRAGVQIAIMTFKDYFYCWNTFVENHERTFKAW EGLHENSVRLSRQLRRILLPLYEVDDLRDAFRTLGL (SEQ ID NO: 51) hAIDv solubility variant lacking N-terminal RNA-binding region MDPHIFTSNFNNGIGRHKTYLCYEVERLDSATSFSLDFGYLRNKNGCHVELL FLRYISDWDLDPGRCYRVTWFTSWSPCYDCARHVADFLRGNPNLSLRIFTARLYFC EDRKAEPEGLRRLHRAGVQIAIMTFKDYFYCWNTFVENHERTFKAWEGLHENSVRL SRQLRRILLPLYEVDDLRDAFRTLGL (SEQ ID NO: 52) hAIDv solubility variant lacking N-terminal RNA-binding region and the C-terminal poorly structured region MDPHIFTSNFNNGIGRHKTYLCYEVERLDSATSFSLDFGYLRNKNGCHVELL FLRYISDWDLDPGRCYRVTWFTSWSPCYDCARHVADFLRGNPNLSLRIFTARLYFC EDRKAEPEGLRRLHRAGVQIAIMTFKDYFYCWNTFVENHERTFKAWEGLHENSVRL SRQLRRILLPL (SEQ ID NO: 53) rAPOBEC1 MSSETGPVAVDPTLRRRIEPHEFEVFFDPRELRKETCLLYEINWGGRHSIW RHTSQNTNKHVEVNFIEKFTTERYFCPNTRCSITWFLSWSPCGECSRAITEFLSRYP HVTLFIYIARLYHHADPRNRQGLRDLISSGVTIQIMTEQESGYCWRNFVNYSPSNEA HWPRYPHLWVRLYVLELYCIILGLPPCLNILRRKQPQLTFFTIALQSCHYQRLPPHIL WATGLK (SEQ ID NO: 54) mAPOBEC3 MGPFCLGCSHRKCYSPIRNLISQETFKFHFKNLGYAKGRKDTFLCYEVTRK DCDSPVSLHHGVFKNKDNIHAEICFLYWFHDKVLKVLSPREEFKITWYMSWSPCFE CAEQIVRFLATHHNLSLDIFSSRLYNVQDPETQQNLCRLVQEGAQVAAMDLYEFKK CWKKFVDNGGRRFRPWKRLLTNFRYQDSKLQEILRRMDPLSEEEFYSQFYNQRVK HLCYYHRMKPYLCYQLEQFNGQAPLKGCLLSEKGKQHAEILFLDKIRSMELSQVTIT CYLTWSPCPNCAWQLAAFKRDRPDLILHIYTSRLYFHWKRPFQKGLCSLWQSGILV DVMDLPQFTDCWTNFVNPKRPFRPWKGLEIISRRTQRRLRRIKESWGLQDLVNDF GNLQLGPPMSN (SEQ ID NO: 55) mAPOBEC3 catalytic domain MGPFCLGCSHRKCYSPIRNLISQETFKFHFKNLGYAKGRKDTFLCYEVTRK DCDSPVSLHHGVFKNKDNIHAEICFLYWFHDKVLKVLSPREEFKITWYMSWSPCFE CAEQIVRFLATHHNLSLDIFSSRLYNVQDPETQQNLCRLVQEGAQVAAMDLYEFKK CWKKFVDNGGRRFRPWKRLLTNFRYQDSKLQEILRR (SEQ ID NO: 56) hAPOBEC3A MEASPASGPRHLMDPHIFTSNFNNGIGRHKTYLCYEVERLDNGTSVKMDQ HRGFLHNQAKNLLCGFYGRHAELRFLDLVPSLQLDPAQIYRVTWFISWSPCFSWGC AGEVRAFLQENTHVRLRIFAARIYDYDPLYKEALQMLRDAGAQVSIMTYDEFKHCW DTFVDHQGCPFQPWDGLDEHSQALSGRLRAILQNQGN (SEQ ID NO: 57) hAPOBEC3G MKPHFRNTVERMYRDTFSYNFYNRPILSRRNTVWLCYEVKTKGPSRPPLDA KIFRGQVYSELKYHPEMRFFHWFSKWRKLHRDQEYEVTWYISWSPCTKCTRDMAT FLAEDPKVTLTIFVARLYYFWDPDYQEALRSLCQKRDGPRATMKIMNYDEFQHCWS KFVYSQRELFEPWNNLPKYYILLHIMLGEILRHSMDPPTFTFNFNNEPWVRGRHETY LCYEVERMHNDTVWLLNQRRGFLCNQAPHKHGFLEGRHAELCFLDVIPFWKLDLD QDYRVTCFTSWSPCFSCAQEMAKFISKNKHVSLCIFTARIYDDQGRCQEGLRTLAE AGAKISIMTYSEFKHCWDTFVDHQGCPFQPWDGLDEHSQDLSGRLRAILQNQEN (SEQ ID NO: 58) hAPOBEC3G catalytic domain PPTFTFNFNNEPWVRGRHETYLCYEVERMHNDTWVLLNQRRGFLCNQAP HKHGFLEGRHAELCFLDVIPFWKLDLDQDYRVTCFTSWSPCFSCAQEMAKFISKNK HVSLCIFTARIYDDQGRCQEGLRTLAEAGAKISIMTYSEFKHCWDTFVDHQGCPFQ PWDGLDEHSQDLSGRLRAILQNQEN (SEQ ID NO: 59) hAPOBEC3H MALLTAETFRLQFNNKRRLRRPYYPRKALLCYQLTPQNGSTPTRGYFENKK KCHAEICFINEIKSMGLDETQCYQVTCYLTWSPCSSCAWELVDFIKAHDHLNLGIFA SRLYYHWCKPQQKGLRLLCGSQVPVEVMGFPKFADCWENFVDHEKPLSFNPYKM LEELDKNSRAIKRRLERIKIPGVRAQGRYMDILCDAEV (SEQ ID NO: 60) hAPOBEC3F MKPHFRNTVERMYRDTFSYNFYNRPILSRRNTVWLCYEVKTKGPSRPRLD AKIFRGQVYSQPEHHAEMCFLSWFCGNQLPAYKCFQITWFVSWTPCPDCVAKLAE FLAEHPNVTLTISAARLYYYWERDYRRALCRLSQAGARVKIMDDEEFAYCWENFVY SEGQPFMPWYKFDDNYAFLHRTLKEILRNPMEAMYPHIFYFHFKNLRKAYGRNES WLCFTMEVVKHHSPVSWKRGVFRNQVDPETHCHAERCFLSWFCDDILSPNTNYEV TWYTSWSPCPECAGEVAEFLARHSNVNLTIFTARLYYFWDTDYQEGLRSLSQEGA SVEIMGYKDFKYCWENFVYNDDEPFKPWKGLKYNFLFLDSKLQEILE (SEQ ID NO: 61) hAPOBEC3F catalytic domain KEILRNPMEAMYPHIFYFHFKNLRKAYGRNESWLCFTMEVVKHHSPVSWKR GVFRNQVDPETHCHAERCFLSWFCDDILSPNTNYEVTWYTSWSPCPECAGEVAEF LARHSNVNLTIFTARLYYFWDTDYQEGLRSLSQEGASVEIMGYKDFKYCWENFVYN DDEPFKPWKGLKYNFLFLDSKLQEILE (SEQ ID NO: 62) S. aureus Cas9 MKRNYILGLDIGITSVGYGIIDYETRDVIDAGVRLFKEANVENNEGRRSKRGA RRLKRRRRHRIQRVKKLLFDYNLLTDHSELSGINPYEARVKGLSQKLSEEEFSAALL HLAKRRGVHNVNEVEEDTGNELSTKEQISRNSKALEEKYVAELQLERLKKDGEVRG SINRFKTSDYVKEAKQLLKVQKAYHQLDQSFIDTYIDLLETRRTYYEGPGEGSPFGW KDIKEWYEMLMGHCTYFPEELRSVKYAYNADLYNALNDLNNLVITRDENEKLEYYE KFQIIENVFKQKKKPTLKQIAKEILVNEEDIKGYRVTSTGKPEFTNLKVYHDIKDITARK EIIENAELLDQIAKILTIYQSSEDIQEELTNLNSELTQEEIEQISNLKGYTGTHNLSLKAI NLILDELWHTNDNQIAIFNRLKLVPKKVDLSQQKEIPTTLVDDFILSPVVKRSFIQSIKVI NAIIKKYGLPNDIIIELAREKNSKDAQKMINEMQKRNRQTNERIEEIIRTTGKENAKYLI EKIKLHDMQEGKCLYSLEAIPLEDLLNNPFNYEVDHIIPRSVSFDNSFNNKVLVKQEE NSKKGNRTPFQYLSSSDSKISYETFKKHILNLAKGKGRISKTKKEYLLEERDINRFSV QKDFINRNLVDTRYATRGLMNLLRSYFRVNNLDVKVKSINGGFTSFLRRKWKFKKE RNKGYKHHAEDALIIANADFIFKEWKKLDKAKKVMENQMFEEKQAESMPEIETEQEY KEIFITPHQIKHIKDFKDYKYSHRVDKKPNRELINDTLYSTRKDDKGNTLIVNNLNGLY DKDNDKLKKLINKSPEKLLMYHHDPQTYQKLKLIMEQYGDEKNPLYKYYEETGNYLT KYSKKDNGPVIKKIKYYGNKLNAHLDITDDYPNSRNKWKLSLKPYRFDVYLDNGVY KFVTVKNLDVIKKENYYEVNSKCYEEAKKLKKISNQAEFIASFYNNDLIKINGELYRVI GVNNDLLNRIEVNMIDITYREYLENMNDKRPPRIIKTIASKTQSIKKYSTDILGNLYEVK SKKHPQIIKKG (SEQ ID NO: 63) C. jejuni Cas9 MARILAFDIGISSIGWAFSENDELKDCGVRIFTKVENPKTGESLALPRRLARS ARKRLARRKARLNHLKHLIANEFKLNYEDYQSFDESLAKAYKGSLISPYELRFRALN ELLSKQDFARVILHIAKRRGYDDIKNSDDKEKGAILKAIKQNEEKLANYQSVGEYLYK EYFQKFKENSKEFTNVRNKKESYERCIAQSFLKDELKLIFKKQREFGFSFSKKFEEE VLSVAFYKRALKDFSHLVGNCSFFTDEKRAPKNSPLAFMFVALTRIINLLNNLKNTEG ILYTKDDLNALLNEVLKNGTLTYKQTKKLLGLSDDYEFKGEKGTYFIEFKKYKEFIKAL GEHNLSQDDLNEIAKDITLIKDEIKLKKALAKYDLNQNQIDSLSKLEFKDHLNISFKALK LVTPLMLEGKKYDEACNELNLKVAINEDKKDFLPAFNETYYKDEVTNPWLRAIKEY RKVLNALLKKYGKVHKINIELAREVGKNHSQRAKIEKEQNENYKAKKDAELECEKLG LKINSKNILKLRLFKEQKEFCAYSGEKIKISDLQDEKMLEIDHIYPYSRSFDDSYMNKV LVFTKQNQEKLNQTPFEAFGNDSAKWQKIEVLAKNLPTKKQKRILDKNYKDKEQKN FKDRNLNDTRYIARLVLNYTKDYLDFLPLSDDENTKLNDTQKGSKVHVEAKSGMLTS ALRHTWGFSAKDRNNHLHHAIDAVIIAYANNSIVKAFSDFKKEQESNSAELYAKKISE LDYKNKRKFFEPFSGFRQKVLDKIDEIFVSKPERKKPSGALHEETFRKEEEFYQSYG GKEGVLKALELGKIRKVNGKIVKNGDMFRVDIFKHKKTNKFYAVPIYTMDFALKVLPN KAVARSKKGElKDWILMDENYEFCFSLYKDSLILIQTKDMQEPEFVYYNAFTSSTVSL IVSKHDNKFETLSKNQKILFKNANEKEVIAKSIGIQNLKVFEKYIVSALGEVTKAEFRQ REDFKK (SEQ ID NO: 64) P. lavamentivorans Cas9 MERIFGFDIGTTSIGFSVIDYSSTQSAGNIQRLGVRIFPEARDPDGTPLNQQR RQKRMMRRQLRRRRIRRKALNETLHEAGFLPAYGSADWPVVMADEPYELRRRGL EEGLSAYEFGRAIYHLAQHRHFKGRELEESDTPDPDVDDEKEAANERAATLKALKN EQTTLGAWLARRPPSDRKRGIHAHRNVVAEEFERLWEVQSKFHPALKSEEMRARI SDTIFAQRPVFWRKNTLGECRFMPGEPLCPKGSWLSQQRRMLEKLNNLAIAGGNA RPLDAEERDAILSKLQQQASMSWPGVRSALKALYKQRGEPGAEKSLKFNLELGGE SKLLGNALEAKLADMFGPDWPAHPRKQEIRHAVHERLWAADYGETPDKKRVIILSE KDRKAHREAAANSFVADFGITGEQAAQLQALKLPTGWEPYSIPALNLFLAELEKGER FGALVNGPDWEGWRRTNFPHRNQPTGEILDKLPSPASKEERERISQLRNPTVVRT QNELRKVVNNLIGLYGKPDRIRIEVGRDVGKSKREREEIQSGIRRNEKQRKKATEDLI KNGIANPSRDDVEKWILWKEGQERCPYTGDQIGFNALFREGRYEVEHIWPRSRSF DNSPRNKTLCRKDVNIEKGNRMPFEAFGHDEDRWSAIQIRLQGMVSAKGGTGMSP GKVKRFLAKTMPEDFAARQLNDTRYAAKQILAQLKRLWPDMGPEAPVKVEAVTGQ VTAQLRKLVVTLNNILADDGEKTRADHRHHAIDALTVACTHPGMTNKLSRYWQLRDD PRAEKPALTPPWDTIRADAEKAVSEIVVSHRVRKKVSGPLHKETTYGDTGTDIKTKS GTYRQFVTRKKIESLSKGELDEIRDPRIKEIVAAHVAGRGGDPKKAFPPYPCVSPGG PEIRKVRLTSKQQLNLMAQTGNGYADLGSNHHIAIYRLPDGKADFEIVSLFDASRRL AQRNPIVQRTRADGASFVMSLAAGEAIMIPEGSKKGIWIVQGVWASGQVVLERDTD ADHSTTTRPMPNPILKDDAKKVSIDPIGRVRPSND (SEQ ID NO: 65) N. cinerea Cas9 MAAFKPNPMNYILGLDIGIASVGWAIVEIDEEENPIRLIDLGVRVFERAEVPKT GDSLAAARRLARSVRRLTRRRAHRLLRARRLLKREGVLQAADFDENGLIKSLPNTP WQLRAAALDRKLTPLEWSAVLLHLIKHRGYLSQRKNEGETADKELGALLKGVADNT HALQTGDFRTPAELALNKFEKESGHIRNQRGDYSHTFNRKDLQAELNLLFEKQKEF GNPHVSDGLKEGIETLLMTQRPALSGDAVQKMLGHCTFEPTEPKAAKNTYTAERFV WLTKLNNLRILEQGSERPLTDTERATLMDEPYRKSKLTYAQARKLLDLDDTAFFKGL RYGKDNAEASTLMEMKAYHAISRALEKEGLKDKKSPLNLSPELQDEIGTAFSLFKTD EDITGRLKDRVQPEILEALLKHISFDKFVQISLKALRRIVPLMEQGNRYDEACTEIYGD HYGKKNTEEKIYLPPIPADEIRNPVVLRALSQARKVINGVVRRYGSPARIHIETAREV GKSFKDRKEIEKRQEENRKDREKSAAKFREYFPNFVGEPKSKDILKLRLYEQQHGK CLYSGKEINLGRLNEKGYVEIDHALPFSRTWDDSFNNKVLALGSENQNKGNQTPYE YFNGKDNSREWQEFKARVETSRFPRSKKQRILLQKFDEDGFKERNLNDTRYINRFL CQFVADHMLLTGKGKRRVFASNGQITNLLRGFWGLRKVRAENDRHHALDAVVVAC STIAMQQKITRFVRYKEMNAFDGKTIDKETGEVLHQKAHFPQPWEFFAQEVMIRVF GKPDGKPEFEEADTPEKLRTLLAEKLSSRPEAVHKYVTPLFISRAPNRKMSGQGHM ETVKSAKRLDEGISVLRVPLTQLKLKDLEKMVNREREPKLYEALKARLEAHKDDPAK AFAEPFYKYDKAGNRTQQVKAVRVEQVQKTGVWVHNHNGIADNATIVRVDVFEKG GKYYLVPIYSWQVAKGILPDRAVVQGKDEEDWTVMDDSFEFKFVLYANDLIKLTAKK NEFLGYFVSLNRATGAIDIRTHDTDSTKGKNGIFQSVGVKTALSFQKYQIDELGKEIR PCRLKKRPPVR (SEQ ID NO: 66) C. lari Cas9 MRILGFDIGINSIGWAFVENDELKDCGVRIFTKAENPKNKESLALPRRNARSS RRRLKRRKARLIAIKRILAKELKLNYKDYVAADGELPKAYEGSLASVYELRYKALTQN LETKDLARVILHIAKHRGYMNKNEKKSNDAKKGKILSALKNNALKLENYQSVGEYFY KEFFQKYKKNTKNFIKIRNTKDNYNNCVLSSDLEKELKLILEKQKEFGYNYSEDFINEI LKVAFFQRPLKDFSHLVGACTFFEEEKRACKNSYSAWEFVALTKIINEIKSLEKISGEI VPTQTINEVLNLILDKGSITYKKFRSCINLHESISFKSLKYDKENAENAKL1DFRKLVEF KKALGVHSLSRQELDQISTHITLIKDNVKLKTVLEKYNLSNEQINNLLEIEFNDYINLSF KALGMILPLMREGKRYDEACEIANLKPKTVDEKKDFLPAFCDSIFAHELSNPWNRAI SEYRKVLNALLKKYGKVHKIHLELARDVGLSKKAREKIEKEQKENQAVNAWALKECE NIGLKASAKNILKLKLWKEQKEICIYSGNKISIEHLKDEKALEVDHIYPYSRSFDDSFIN KVLVFTKENQEKLNKTPFEAFGKNIEKWSKIQTLAQNLPYKKKNKILDENFKDKQQE DFISRNLNDTRYIATLIAKYTKEYLNFLLLSENENANLKSGEKGSKIHVQTISGMLTSV LRHTWGFDKKDRNNHLHHALDAIIVAYSTNSIIKAFSDFRKNQELLKARFYAKELTSD NYKHQVKFFEPFKSFREKILSKIDEIFVSKPPRKRARRALHKDTFHSENKIIDKCSYNS KEGLQIALSCGRVRKIGTKYVENDTIVRVDIFKKQNKFYAIPIYAMDFALGILPNKIVIT GKDKNNNPKQWQTIDESYEFCFSLYKNDLILLQKKNMQEPEFAYYNDFSISTSSICV EKHDNKFENLTSNQKLLFSNAKEGSVKVESLGIQNLKVFEKYIITPLGDKIKADFQPR ENISLKTSKKYGLR (SEQ ID NO: 67)

Example 1. Improving the Genome-Wide Specificities of Targeted Base Editing Technologies

Wild type cytidine deaminase domains have intrinsic substrate sequence specificity for 2-3 nucleotide motifs (see Table 1). APOBEC enzymes recognize their cognate sequence motifs through direct contacts formed between residues in two recognition loops with variable sequence composition termed recognition loop 1 (RL1) and Loop 7^(12-13, 17-22) (see Table 2, FIGS. 2A-2B, and FIGS. 3A-3B). For instance, the primary determinant of APO3A substrate sequence specificity is residue D131 in loop 7, which makes two hydrogen bonds with the thymine in the 5′ TC motif. To modify the specificity of APO3A we altered the identity of the residue at position 131 to residues that have previously been demonstrated to form base-specific contacts (Table 7)⁴³.

To alter the specificities of all other APOBEC deaminases listed in Table 1, we altered homologous residues from each of these proteins identified by sequence alignment to APO3A.

Although wild type APO3A possesses intrinsic sequence specificity for the 5′ TcR motif, it is able to deaminate 5′ AcR, 5′ GcR and 5′ CcR motifs (wherein the lowercase C is the base that is deaminated) with lower efficiencies. This suggests that it might be possible to engineer APO3A to have greater specificity for its canonical TcR substrate motif by removing excess binding energy in the form of contacts made between APO3A and its substrate ssDNA such that only TcG or TcA motifs are efficiently deaminated. Based on the crystal structure of APO3A bound to substrate ssDNA²¹, we identified R28 and K30 (which seem to contact the base immediately 3′ of the TCR in a semi-specific manner) as well as N57, R60, and Y130 (which all contact the ssDNA substrate backbone in non-base-specific manners) as candidate residues whose DNA contacts might contribute significant non-specific substrate binding energy such that altering them may result in a more specific deaminase. We have also identified W98 as a residue that contributes to the formation of the hydrophobic pocket that the target cytosine base is buried in and hypothesized that W98Y would decrease the hydrophobicity of this pocket while retaining deaminase activity, thereby decreasing any possible excess binding energy above that which is required for deamination of the Tc motif. Because multiple APOBEC homologs and orthologs bear significant similarity to APO3A at the sequence level, we have also identified the cognate residues expected to increase substrate sequence specificity for each of these proteins.

To validate the mutations we hypothesized as able to contribute to engineered base editing proteins with increased specificity for TcR motifs, we cloned genes encoding APO3A-nCas9-UGI proteins bearing single residue substitutions in APO3A into plasmid vectors for protein overexpression in mammalian cells. We then transfected these plasmid vectors into human U2OS cells in combination with a plasmid designed to express a guide RNA targeting one of two discrete sites bearing multiple trinucleotide motifs capable of acting as deamination substrates within a single integrated EGFP gene. After 72 hours, we harvested the genomic DNA from the transfected U2OS populations and performed high-throughput amplicon sequencing at the guide RNA genomic target sites. We found that when the wild-type APO3A protein was fused to nCas9-UGI, the resulting protein was able to effect C>T transitions at the target site at the expected TcR motifs, but also on the GcT and GcG motifs present at the target site (FIG. 4). However, when N57 or K60 were mutated to residues listed in Table 7 we observed robust deamination at the on-target TcG present in the editing window, but significantly decreased ability of these proteins to target motifs outside of the base editing window and at the GcT motif present in the editing window. Notably, APO3A N57A-nCas9-UGI and APO3A K60D-nCas9-UGI retained robust activity on the editing window TcG but low to very low activity on the editing window GcT motif (FIG. 5). To quantitatively evaluate this increase in preference for TcG over GcT in the editing window of EGFP site 1, we divided the deamination frequency at the TcG motif in the editing window by the deamination frequency of the GcT motif in the editing window to determine the specificity ratio for these proteins (FIG. 6). We found that APO3A N57A-nCas9-UGI and APO3A K60D/E/N-nCas9-UGI both had demonstrated increased specificity for the TcG motif over the GcT motif relative to the wild-type APO3A-nCas9-UGI protein. We then evaluated the activity of these proteins on EGFP site 2, a target site bearing GcA, AcA, AcC, CcC, CcA and GcG motifs as well as a TcG motif outside of the editing window. Each of the previously described mutants demonstrated significantly decreased activity on each of these off target motifs in the target site, as well as decreased activity on the on-target TCG motif found outside of the editing window (FIG. 7, FIG. 8).

Based on the crystal structure of APO3A in complex with its ssDNA substrate²¹, we determined that K30 makes a base-specific contact to the third nucleotide in the TcG motif. We hypothesized that mutations made to the residue at this position will alter the identity of the third nucleotide recognized by APO3A-nCas9-UGI in a trinucleotide substrate motif. Because we expect the identity of the residue at position 30 to significantly influence the identity of the third nucleotide in a substrate motif, we have determined the residues we expect to make contacts to specific bases in Table 9.

Mutations made to various residues in APO3A (Table 9) were able to restore sequence preference to varying degrees when the resulting plasmid DNAs encoding base editor proteins were delivered by transient transfection to human cells along with a plasmid encoding a gRNA targeting a chromosomally-integrated EGFP gene (FIG. 9). We also found that we could enhance the specificity of APO3A for its cognate 5′TC motif by combining mutations to positions listed in Table 7 and targeting the BE proteins to the same EGFP target site by transient transfection of plasmid, resulting in the APO3A N57Q/Y130F BE3 variant (FIG. 10) which had similar motif specificity to APO3A N57A or N57G BE3.

We screened the activities of APO3A N57A or N57G or (N57Q/Y130F) BE3 at 12 endogenous genomic sites that contained a cognate 5′TC motif in the editing window in addition to another, non-cognate 5′VC (where V=A, C, or G) and compared them to BE3 and the state-of-the-art engineered variants YE1, YE2, and YEE BE3 (YE BE3s), which decrease the frequency of such bystander mutations by incorporating point mutations into the rat APOBEC1 (rAPO1) deaminase domain that slow its kinetic rate and limit the length of its editing window compared to BE3⁴⁴. We found that at 8 of the 12 sites, the engineered APO3A BE3 variants induced C-to-T editing at cognate motifs 5- to 264-fold more than at the non-cognate 5′VC motifs (FIGS. 11-12). However, the engineered APO3A BE3 variants induced cognate:non-cognate editing at ratios less than 5 at the remaining 4 sites.

We next sought to improve sequence-specific deamination at these sites by adding mutations to APO3A N57G BE3 at residues previously shown to influence the catalytic rate and processivity of homologous proteins (Table 10). Although the addition of the individual homologous mutations derived from the YE BE3 proteins did not significantly increase sequence specificity of the APO3A N57G double mutants, mutations made to residues A71 and 196 greatly increased the cognate:non-cognate editing ratios for the three tested sites from less than 5 to approximately 13 (FIG. 13).

Critically, the exact nature of these mutations may differ in a manner dependent on delivery modality. For instance, delivery of these reagents by ribonucleoprotein (RNP) or encoded in mRNA may result in shorter duration of the proteins in cells. Shorter duration of base editor proteins in cells can result in different mutational spectra compared to longer-lived delivery, e.g. by plasmid transfection⁴⁵. As a result, it may be necessary to use engineered cytidine deaminase BEs that retain sub-optimal sequence specificity when delivered by plasmid but optimal sequence specificity when delivered by shorter-lived modalities, for instance APO3A N57Q or K60D or Y130F BE3.

The engineered variant APO3A N57G BE3 also demonstrates increased genome-wide fidelity at off-target sites compared to wild-type APO3A BE3 and BE3. We transiently transfected cells with plasmid DNA encoding APO3A BE3, APO3A N57G BE3, or BE3 along with plasmid that expresses the well-characterized EMX1 (FIG. 14) or FANCF gRNAs (FIG. 15). BE3 induced detectable editing by high-throughput sequencing at 16 of the 25 previously-identified off-target sites for EMX1 and at 3 of the 15 off-target sites for FANCF. Conversely, APO3A N57G BE3 induced editing at 6/25 sites for EMX1 and 0/15 for FANCF. At sites that APO3A N57G BE3 did induce off-target editing at, it was at greatly reduced frequencies compared to BE3. Addition of the high fidelity mutations from HF1 or Hypa SpCas9 to APO3A N57G BE3 reduced all off-target editing to below the detection threshold of the assay.

Finally, we sought to determine whether APO3A N57G base editors could be used to more efficiently correct the beta-thalassemia mutation HBB −28 (A>G). In the gRNA targeting this mutation (CTGACTTcTATGCCCAGCCC (where the bolded lowercase “c” is the target cytosine)) on the antisense strand, a second cytidine preceded by a 5′A (bystander cytidine) exists in the editing window in addition to the target cytidine preceded by a 5′T at position −28. Mutation of the bystander cytidine produces independent beta thalassemia phenotypes and should be avoided in any potential therapy for the HBB −28 (A>G) mutation. We transiently transfected plasmid DNA encoding BE3 or APO3A N57G BE3 (as well as the other proteins shown in FIG. 16, including those adding a second UGI domain to A3A N57G BE3 or the HF1 or Hypa high fidelity mutations to the nCas9 moiety) into HEK293 cells bearing a lentivirally-integrated 200 base pair fragment of the HBB promoter encoding the HBB −28 (A>G) mutation. After 72 hours, we harvested genomic DNA from the cells and used PCR to amplify the target site, and sequenced the PCR product by illumina high throughput sequencing to examine the deamination frequencies of both the target and bystander cytidines.

We found that, while the BE3 and YE BE3 proteins edited both the target and bystander cytidines at approximately equal rates, APO3A N57G BE3 deaminated the target cytidine approximately 15-fold more than the bystander cytidine (FIG. 17). We then analyzed the frequency with which editing the HBB −28 (A>G) site with BE3 produced perfectly corrected alleles (i.e. alleles in which the −28 position has been edited but not any other position). BE3 produced perfectly edited alleles at a frequency of 0.5% of the total alleles sequenced. Conversely, editing with APO3A N57G BE3 produced perfectly edited alleles at a rate of 22% of total sequenced alleles, 40-fold more than editing with BE3 (FIG. 18). We next investigated whether editing with APO3A N57G BE3 produced fewer off target deamination events than BE3 using the HBB −28 (A>G) gRNA. BE3 induced detectable deamination at 3 of 8 off target sites, while APO3A N57G BE3 induced editing at just 1 of the 8 off target sites (FIG. 19). Thus, the engineered APO3A N57G BE3 variant is able to more efficiently correct the HBB −28 (A>G) disease-causing allele with fewer off-target effects at the eight examined sites as compared to BE3 or the state-of-the-art YE BE3 proteins.

In additional experiments, a human patient's CD34+ HSPCs that have the HBB −28 (A>G) mutation that are harvested from donation are used. Purified A3A N57G BE3 protein is delivered with guide RNA to the cells. After 5 days, gDNA is extracted and disease correction is evaluated by sequencing.

The mutations listed in Table 7 can be used to increase specificity of deaminase proteins or domains on their own or in any possible combinations. The mutations listed in Table 8 are intended to alter the targetable motif sequence, and can be combined with any of the mutations in Table 7 to create engineered deaminase proteins or domains with altered and increased substrate sequence. Further, the mutations listed in Table 9 can be combined with any of the mutations listed in Table 7 or Table 8 to create engineered deaminase proteins with altered specificity for the first or third nucleotide in a trinucleotide motif and with increased specificity for its target motif relative to other possible deamination substrate motifs.

TABLE 7 Mutations that enhance the sequence specificity of cytidine deaminase proteins by affecting the protein:substrate interaction interface Mutation 1 2 3 4 5 6 APO3A R28A, K30A, N57A, G, K60A, D, W98Y Y130A or F E, or Q E, or Q D, E, K, E, R, N, Q, or S or Q rAPOBEC1* — E22A, S49A, G, R52A, D, W90Y Y120A or F K, or Q D, E, K, E, K, N, or Q or Q mAPOBEC3* R39A, D41A, N66A, G, D68A, E, W102Y Y132A or F E, or Q E, or Q D, E, K, R, N, or Q, or S Q hAPOBEC3C R30A, E32A, N57A, G, D60A, E, W94Y Y124A or F E, or Q D, or Q D, E, K, R, N, or Q, or S Q hAPOBEC3G R215A, E217A, N244A, — W285Y Y315A or F E, or Q D, or Q G, D, E, K, Q, or S hAPOBEC3H* — R21A, N49A, D, K52A, D, W82Y Y112A or F OR E, or Q E, K, Q, E, R, N, Y113A or F or S or Q hAPOBEC3F R213A, E215A, N240A, D243A, W277Y Y307F E, or Q K, or Q D, E, K, E, R, N, Q, or S or Q *indicates which proteins lack sufficient structural information. Table 7 shows APOBEC orthologs with significant sequence and structural similarity for specificity engineering. In these cases, protein sequence alignment to APO3A was used to determine the residue homologous to the APO3A position. Each of the six residue positions to be mutated are listed with one or more residues that are expected to increase the specificity of that deaminase domain for its canonical or re-engineered substrate sequence by reducing excess binding energy between the deaminase protein and its ssDNA substrate.

TABLE 8 APOBEC orthologs with significant sequence and structural similarity for substrate sequence specificity re-engineering (* indicates which proteins lack sufficient structural information. Substrate motif 5′-TC 5′-GC 5′-AC 5′-CC APO3A 5′-TCR D131 D131R/K D131N/Q/R D131E/H/S mAPOBEC3* 5′-TYC N133D N133R/K N133Q/R N133 E/H/S hAPO3B 5′-TCR D314 D314R/K D314N/Q/R D314 E/H/S hAPOBEC3C 5′-YC Y125D Y125R/K Y125N/Q/R Y125 E/H/S hAPOBEC3G 5′-CCC D316 D316R/K D316N/Q/R D316 hAPOBEC3H* 5′-TC H114 H114R/K H114N/Q/R H114E/S hAPOBEC3F 5′-TC Y308 Y308R/K Y308N/Q/R Y308 E/H/S *indicates which proteins lack sufficient structural information. In these cases, protein sequence alignment to APO3A was used to determine the residue homologous to APO3A D131. The position and identity of the residue mutations expected to alter each protein's sequence specificity are given for each two-nucleotide motif. All positional information refers to the wild-type protein sequences acquired from uniprot. org.

TABLE 9 APOBE orthologs with significant sequence and structural similarity for substrate sequence specificity re-engineering 5′-NCA 5′-NCG 5′-NCT 5′-NCC APO3A K30N/Q/R K30R K30D/E/R K30D/E/H/S hAPO3B* Q213N/R Q213R/K Q213D/E/R Q213D/E/H/S *indicates which proteins lack sufficient structural information. In these cases, protein sequence alignment to APO3A was used to determine the residue homologous to APO3A K30. The position and identity of the residue mutations expected to alter each protein's sequence specificity are given for each three-nucleotide motif. All positional information refers to the wild-type protein sequences acquired from uniprot.org.

TABLE 10 Mutations that enhance the sequence specificity of cytidine deaminase proteins by affecting the kinetic rate and processivity of the enzyme. Mutation 1 2 APO3A A71G, V, I, I96T, S, A, L, S, or T V, L, M, or G rAPOBEC1 V62G, V, I, L88T, S, A, L, S, or T V, I, M, or G mAPOBEC3 A72G, V, I, M100T, S, A, L, S, or T V, I, L, or G hAPOBEC3C A67G, V, I, T92S, A, V, I, L, S, or T L, M, or G hAPOBEC3G A258G, V, I, T283S, A, V, I, L, S, or T L, M, or G hAPOBEC3H A55G, V, I, L80T, S, A, V, L, S, or T I, M, or G hAPOBEC3F A250G, V, I, T275S, A, V, I, L, S, or T L, M, or G

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OTHER EMBODIMENTS

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. 

What is claimed is:
 1. A fusion protein comprising a Cas9-like nickase (nCas9) linked to an engineered variant deaminase hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H, with an optional intervening linker, comprising one or more mutations shown in Table 7, 8 and/or 9 in the substrate sequence motif of the deaminase that causes the deaminase to act at a non-canonical motif, and optionally wherein the fusion protein further comprises a uracil glycosylase inhibitor (UGI).
 2. The fusion protein of claim 1, wherein the fusion protein comprises an engineered variant of a deaminase domain listed in Table 1 or Table 2 that recognizes a non-cognate sequence motif.
 3. The fusion protein of claim 1, wherein the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises one or more mutations shown in Tables 7 or
 8. 4. The fusion protein of claim 1, wherein the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises (i) one or more mutations shown in Table 7 and/or 8 and (ii) one or more mutations shown in Table
 9. 5. The fusion protein of claim 1, wherein the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H comprises hAPOBEC3A with a mutation at one or more of N57; K60, and/or Y130); or hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3G, or hAPOBEC3H with a mutation corresponding to N57, K60, or Y130.
 6. The fusion protein of claim 1, wherein the engineered variant comprises hAPOBEC3A with a mutation at N57 and Y130.
 7. The fusion protein of claims 5-6, comprising hAPOBEC3A with one or more of a N57G or N57Q mutation; a K60A or K60D mutation; and/or a Y130F mutation.
 8. The fusion protein of claim 5, wherein the engineered variant comprises hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3Cc or hAPOBEC3H with a mutation corresponding to N57, K60, and/or a mutation corresponding to Y130.
 9. The fusion protein of claim 5 or 7, comprising hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3Cc or hAPOBEC3H with one or more of a mutation corresponding to N57G; K60D; or Y130F.
 10. The fusion protein of claims 1-9, wherein the engineered variant of hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3A, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3Cc or hAPOBEC3H comprises hAPOBEC3A with a mutation at A71 and/or 196, or hAID, rAPOBEC1*, mAPOBEC3, hAPOBEC3B, hAPOBEC3C, hAPOBEC3F, hAPOBEC3Cc or hAPOBEC3H with a mutation corresponding to A71 and/or 196 (e.g., as shown in table 10).
 11. A method of treating a subject with beta thalassemia mutation HBB-28 (A>G), the method comprising delivering a therapeutically effective amount of a fusion protein of any of the preceding claims, wherein the deaminase comprises APO3A comprising a mutation at N57G or N57A or N57Q or K60A or K60D or Y130F, and preferably wherein the fusion protein comprises a ssDNA nicking or catalytically-inactive Cas9.
 12. The method of claim 11, wherein the fusion protein is delivered as an RNP, mRNA, or plasmid.
 13. The method of claim 11, comprising delivering the fusion protein ex vivo to a population of cells comprising CD34+ hematopoietic stem and/or progenitor cells collected from the subject under conditions sufficient for deamination of the mutated, and re-infusing the cells back into the subject.
 14. A method of deaminating a selected cytidine in a nucleic acid, the method comprising contacting the nucleic acid with a fusion protein or base editing system of any of claims 1-10.
 15. A composition comprising a purified a fusion protein or base editing system of any of claims 1-10.
 16. The composition of claim 15, comprising one or more ribonucleoprotein (RNP) complexes.
 17. A nucleic acid encoding a fusion protein or base editing system of any of claims 1-10.
 18. A vector comprising the nucleic acid of claim
 17. 19. An isolated host cell comprising the nucleic acid of claim
 18. 20. The host cell of claim 19, which is a stem cell.
 21. The host cell of claim 19, which is a hematopoietic stem cell. 